This patch adds a `ct_extra_params` option to rebar.config, the value
of which is appended to the shell command when executing common test.

This patch allows the 'suite' argument to eunit to be a comma separated
list of modules to test instead of being a single module. This allows
fine-grained testing when one test suite interferes with another and its
not clear which suite is causing the problem. It also lets you run the
test suite in a different order for a similar reason.

The other enhancement is to add a new eunit parameter; 'skip_app' which
like 'app' is a comma separated list of modules to skip testing on. This
parameter is only applied if the app parameter is not passed. Its
purpose is to avoid forcing you to specify all the apps to test if you
only want to skip a handful and there are many apps to test.

Add no_debug_info option.
Remove debug_info=1 option.

Xref not being stopped caused a crash in subsequent xref runs.

This change makes it possible to assign pre/post scripts to all
rebar commands. This allows users fine grained control over
when scripts and/or shell commands should be executed, where such
extensions are absolutely needed.

Several examples have been added to the rebar.config.sample file.

Currently with default simplenode template, `appname console other_args`
will pass ["console", "other_args"] as plain arguments. But
`appname start other_args` effectively calls `appname console`. This
patch replaces this call with `appname console other_args`.

This commit changes how rebar determines which apps have been
updated, added and removed from a release during appup generation.
Rather than use app files it now determines this from the rel file
in each version of the release. In addition it fixes a bug reported
on the mailing list when generating appups when an application has
been added or removed from either release.

Rebar will exit with {error,bad_directory} when trying to restore the code
path after it has finished working on a subdirectory if there are invalid
relative paths in it. The problem was seen when executing the last line of
rebar_erlc_compiler:doterl_compile/3 (true = code:set_path(CurrPath)).

This change adds support for executing ct test runs based on test
specificiations, which was missing previously. The rebar_ct module
now looks for any number of files with a name ending in `test.spec`
and if it finds one or more, passes these after the `-spec` argument
to ct_run instead of explicitly configuring the config, user config
and coverage config variables.

When no specifications are found, then the module behaves as it did
before this change, and both the ct1 and (new) ct2 integration tests
appear to show this is a backwards compatible patch.

`init:get_plain_arguments()` returns `["console", "more_args"]` when
started with `$APP_NAME console more_args`, but `["console"]` when
started with `$APP_NAME start more_args`. This patch makes `start`
behave like `console` in this respect.