dialyzer_reference plus one minor patch

Document 'rebar help <CMD1> <CMD2>'

change cover html generation to produce valid html

Fixes and updates

- restore support for "rebar help xref"
- update rebar.config.sample
- update 'help xref' string
- simplify new/changed functions by breaking out code or
  using simpler syntax where applicable

- refactor plugin dirs code to be simpler and easier to read
- use erlang-mode's default (%%) comments for portability/consistency
- make sure erlang-mode's indenter is used so that a future whole
  buffer indent doesn't get messed up

Also fix a single line exceeding max column.

Add one more "unicode" option in expand_env_variable—I have a bullet in my prompt that was blowing it up

Allow the use of absolute paths in the lib_dirs configuration setting

Conflicts:
	src/rebar_xref.erl

rebar_eunit:reset_after_eunit/1 mustn't remove 'included_applications'

External deps_dir should have higher priority than the config one

The external deps_dir should have higher priority because
it is used by scripts and other build tools to set up the
location of the dependencies. This commit ensures that,
even if a project has set deps_dir in its config file has
lower preference than the command line one.

Add latest version getopt that automatically wraps usage output lines

Fix file handling on windows for tests

Correct ext of neotoma output file

Ensure we always have a `registered` value in `.app` files

Allow specifying eunit suite run order in 'suites'

Implement merged overlays

Remove leading empty line from a template

Sync test/upgrade_project/rel/files with priv/templates

Use "pwd -P" to get the current physical path.

Support for QuickCheck Mini

Add support for ping command on windows in the simple node runner

It is a portable version of the realpath(1) utility that you can find on
Mac OS X and FreeBSD (see also The Open Group Base Specifications Issue
6, IEEE Std 1003.1).

Without the -P flag, pwd(1) might return different values when the
current path contains one or more symlinks, depending on how you got
into the current directory.

In simplenode.runner, this may cause PIPE_DIR to have different values
on each use, which will make it impossible to connect to the running
node unless you guess the correct path yourself.

Fix searching for plugins

If a plugin is in a dependency, rebar didn't search for it carefully
enough.

Add rebar-deps env to port compiler SharedEnv

REBAR_DEPS_DIR is often needed when a nif needs to be linked with a raw
dependency.

Travisci fixes