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.

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.

--HG--
extra : rebase_source : 5035f025b5

--HG--
extra : rebase_source : cd143fdf55

--HG--
extra : rebase_source : c0bfcfbbbc

--HG--
extra : rebase_source : 1998dc102f

--HG--
extra : rebase_source : 010907d47b