Repository#supported_types refactored (just git is supported for now)
GitDownloader checking if the scm is available
Downloaders::Base#retrieve! (pending a test case)
Finished Downloader::Base retrieve! unit tests
started GitDownloader retrieve! methods
Included missing semicolon.

signed-off-by: Rafael Reggiani Manzo <rr.manzo@gmail.com>
GitDownloader#retrieve!