FInish milestones in projects module.

Closes #11.
Add missing property for milestones.
Add repository files CRUD.

Closes #6.
Fix spelling error in method name.
Add commits to projects module.

Closes #7.
Update README with API URL.
Add top-level issues.
Properly import issues.
Add issues to projects module.

Closes #10.
Add deploy keys to projects module.

Closes #13.
Add system hooks module.

Closes #14.
Initial (unfinished) group implementation.
Finish groups module.

Closes #15.
Make RequestType a normal class, not an enum.

This removes the restriction of needing Python >= 3.4. RequestType is
now a normal class with class variables (fixes #19).