Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

Remove experimental flag from:
* qc compile_only=true
* eunit compile_only=true
* src/foo.app.src.script
* eunit tests=

This allows an <app-name>.app.src.script to be defined and evaluated
when <app-name>.app.src or <app-name>.app are loaded. This allows the
user to add project specific manipulations to app metadata.

This patch adds support for customising the way in which rebar generates
version numbers for app.src files using the `{vsn,Spec}` approach.
Whilst the existing `{vsn,ScmName::atom()}` syntax will continue to
work, users can also pass `{vsn,{cmd,Cmd::string()}}` in which
case the provided *command* will be used. For example:

```erlang
{application, doodah,
 [
  {vsn, {cmd, "git rev-parse --short HEAD"}}]}.
```

Allows using tools like git-subtree and still maintaining proper
git-based vsns for those "imported" subtrees. Also helps understanding
what was the last change introduced to a specific application within a
multi-application repository (at this moment, all applications that
reside in the same repository, will share the same autogenerated git
vsn)

Replace rebar_util:get_cwd/0 by rebar_utils:get_cwd/0.  Luckily the
misspelt module name doesn't seem to have caused any harm, since
rebar_app_utils:is_app_dir/0 and rebar_rel_utils:is_rel_dir/0 aren't
called (only their /1 counterparts).

--HG--
extra : rebase_source : 05a045be5a