civiprj - Add git caching helpers:

 * git_cache_setup - Creates or updates a local mirror of a remote repo
 * git_cache_deref_remotes - Find any working-copies based on the local mirror and configures a proper remote
civiprj - Prefetch common git repos
civiprj - Download civicrm-*.git via cache
Merge pull request #7 from civicrm/master-cache

Implement git caching