Add 'make-snapshot' action

make-snapshot produce tree with all dependencies included, ready to package
to tarball and/or to .src.rpm
Create deps/*/priv/vsn.git with proper versions
Allow to put additional files to snapshot.

I assume that files pulled by script, on 'get-deps'/'update-deps' phase.
(this change was added with yokozuna's jars in mind)
Use rebar_utils:sh/2 directly for "git describe"

because we need to handle errors
Stub cmd_in_dir into built-in rebar functionality
Rename sha_for_project/1 to source_for_project/1

Because it return not only sha1 of project, but also url and vcs type
Refactor \n striping in same way, as in rebar itself
Own functions for get each type of info from git