Add xref target, with exclusions

Remove the workaround for the bug when printing empty binaries in W mode

The fix for this was merged in R16 somewhere, so this workaround is no
longer needed, and fixes the EQC test from spotting the divergent
behaviour with io_lib.

See also: http://erlang.org/pipermail/erlang-patches/2012-September/003034.html

Fix dialyzer warnings and make all the records typed

Rework how dialyzer PLTs are built and used

This commit splits the PLTs into 2, one for all the required OTP
applications that are in the stdlib, and the other for the rebar
dependancies. Each one is created if it does not exist, then checked for
validity and then --add_to_plt is used to add any missing files (this is
very fast if nothing needs to be added). Then the application is
dialyzed using both PLTs.

The 'combo' PLT which resides in ~ is intended to be used by all of
Riak's deps, so it can grow to cover the set of OTP applications that
Riak depends on. Each project can only specify the ones it cares about.

Allow PLT destination to be specified

Fixed formatting.

Fix lager eunit initialization

Set source encoding so R17 compiler doesn't choke on lager_file_backend

Update rebar to support REBAR_DEPS_DIR

Fix doc of lager_console_backend

Clarifying how to disable 'crash_log'

The current set of instructions mention 'crash_log' being left undefined
as a variable to disable it, but when that happens, lager starts it at a
default location.

This commit makes it obvious that the value must explicitly set to
'undefined' in order to disable crash_log.

Update README: fix no_file error and config format error

Can't disable periodic file rotation

Add `async_threshold_window` option. #147

Removed incorrect ']}' from example config.

Documentation spelling fix.