Write git commit ref at top of compiled js during collect_coldbrew

[jMyles]

If collect_coldbrew is run on a project which is tracked by a git repository, let's write the commit ref of the most recent revision of each coffee file to the top of the respective javascript.

For example,

"Brewed from my-script-file.coffee 39fdf3fjew9fwe0fwej33f23fwef"