Travis needs a higher timeout for the async test

Crude but effective fix for #393

* A side effect is you can set per processes functions via lager:md/1

* Updated README with instructions on how to use
 * Basic set of tests for working and non working examples

fix travis badge url

This allow functions to return a value to used in the metadata for tracing
or formatting.

Fix async_threshold_test_ and trace_files_test_ flappiness.

Bugfix and separate test files directory.

Allow Metadata-only logging

Add an uppercase severity formatter

Add config for suppress supervisors start/stop

Make "it" unambiguous

log the ranch error msg

Update TravisCI for OTP 18.3 and 19.0

It wasn't clear whether "it" referred to async_threshold or
async_threshold_window.

On final analysis, it appears the effective fix is clearing out leftover lager state in the application controller.
Along the way, the test body was modified to account for the runtime system, and as this doesn't seem like a bad thing it's left in place.

This commit also moves `otp_version/0` to `lager_util`'s public API, as it may be helpful in future tests that check for version-specific messages (see issue #383).

Uses lager_util:create_test_dir/0 and lager_util:delete_test_dir/1 to give tests, or groups thereof, their own clean directory to work in.
Turn off whitespace diffs to review, as much code is shifted left to allow sane wrapping within around 100 columns.

Change rotate_file_test/0 and rotate_file_fail_test/0 to use temporary test directories.
Refactor rotate_file_test/0 to be marginally understandable.
Fix rotate_file_fail_test/0 to use 'chmod', as intended, instead of 'chown'.

This allows users to log metadata (tuple list) without a format string in lager. For
example `lager:info([{foo, bar}])` instead of `lager:info([{foo,bar}], "")`.

Fix dialyzer warnings

Add `| undefined` to records that get initialized with not all their
fields set to the proper type (hence get the value undefined). These
fixes allow lager to exist as a dependency in a project and be filtered
through that project's dialyzer run without warnings.

BORS seems to be out to lunch - going to merge manually after rebasing on top of master

Restore backwards-compatible `lager_file_backend` configurations.

Reviewed-by: macintux

mark for no crash log

Reviewed-by: JeetKunDoug

The strict configuration work in 31cf6c0 prevents some
backwards-compatible code in lager_file_backend from working as the
configuration is rejected before it gets to the backend. This fix adds
an additional guard on `check_handler_config/2` to support the `tuple`
configuration structure supported by `lager_file_backend`.

Fixed legacy support for {crash_log, undefined}

Reviewed-by: JeetKunDoug

Introduces the option to have severity logged in uppercase letters, as is usual for other programs logging to syslog, for example.

Applications that explicitly configure lager's `crash_log` with the
value `undefined` will get a really terrible crash message for all
versions since 81222ca486

Diff here: 81222ca486 (diff-25e915f0df09d04052d2a6e85b8824f2)

That message looks like this:

```
** Generic server lager_crash_log terminating
** Last message in was rotate
** When Server state == {state,undefined,
                              {file_descriptor,prim_file,{#Port<0.1340>,15}},
                              281571,65536,10485760,
                              [{hour,0}],
                              5,false}
** Reason for termination ==
** {badarg,[{erlang,'++',[undefined,".3"],[]},
           {lager_util,rotate_logfile,2,
                       [{file,"_build/default/lib/lager/src/lager_util.erl"},
                        {line,220}]},
           {lager_crash_log,handle_info,2,
                            [{file,"_build/default/lib/lager/src/lager_crash_log.erl"},
                             {line,99}]},
           {gen_server,try_dispatch,4,[{file,"gen_server.erl"},{line,601}]},
           {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,667}]},
           {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
```

This fix will allow people who use this legacy configuration to not
experience production crashes at midnight after the deploy, when
crash_log tries to rotate for the very first time, resulting in trying
to append `.Int` to `undefined`