Tidy up unimplemented
Re-implement POST /repos/:owner/:repo/git/trees

This reverts commit d268e4813241fd6d4dfd4d68750673f315c1133c and partially
3b0d75ef3bd1d8bcf2ac169ef697d0ae61230793.
Write a basic cook-book for raw git objects (#229)

This is not a high-quality commit. Quality will improve when I write
the associated integration tests to restore 100% coverage.
Implement more git_xxx things (#229)

Still no test coverage :-/
Todo
Add Tag, Commit and Branch (#229)
Test getters of git objects in Repository (#229)
Dramatically improve test coverage for git objects (#229)