Add support for git repositories in the 'includes' directive.

Now, the 'includes' directive supports the following sentences:

includes[remote][makefile] = 'drupal.make'
includes[remote][download][type] = "git"
includes[remote][download][url] = "git@github.com:organisation/repository.git"
; branch could also be tag, code relays on make_download_git function
includes[remote][download][branch] = "7.x"