Add git versions of internal dependencies

I've split up requirements/base.txt into internal and external
dependencies to do this, with svn and git versions of the local deps to
avoid duplicating definitions.

requirements/local_git.txt can be used to install everything with git
versions of internal deps.

git-svn-id: https://source.caret.cam.ac.uk/svn/projects/timetables/trunk@25027 59899196-baf2-4bc4-a94d-4a08719e171f