Avoid redundant unpacking of bundles (from pwaller)
Fixed issue #32, #150, #161 - Fixed checking out the correct tag/branch/commit when updating an editable Git requirement.
Fixed issue #49 - Added ability to install version control requirements without making them editable, e.g.:
pip install git+https://github.com/pypa/pip/
Fixed issue #175 - Correctly locate build and source directory on Mac OS X.
Added git+https:// scheme to Git VCS backend.
Thanks Ronny Pfannschmidt and Wil Tan.