Beginnings of refactoring repository handling code

Moving to a driver-based architecture where we can
support multiple server backends.
GitHub driver connects correctly
Migrated all the GitHub code to the new chisubmit.repos package

Note: The CLI code still points to the old GitHub code.
Added command to create instructors
Finished migrating to new git handling code

Passes full_example.sh
Merge branch 'repo-refactor' into dev