I've added variable expansion in replace overlay

Different fix for the OS environment vs Default environment

The logic changes from strict overrides to a hybrid for merging os_env
and defaults, whereby defaults are chosen if they are not set in the
os_env or if the defaults contains substitutions (at which point os
environment is substituted).

This still means that rebar.config works as it did before, where it
overrides or substitutes based on the use of $VAR, but that default
and os environment merging works such that the common cases of
providing CC or LDFLAGS in the users environment or command line work
as you might expect.  In that CC is overriden by the os environment
unless rebar.config overrides it, and LDFLAGS from the user environment
is appended or prepended based on defaults and rebar.config.

Addresses problems with https://github.com/basho/rebar/pull/71
Also keeps the fix for bug 255.

Allow rebar to use dialyzer plt files from an alternate directory
through the use of the REBAR_PLT_DIR environment variable.

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.