Trees API

Impl / tests for Trees API, http://developer.github.com/v3/git/trees

Why trees? I just picked an useful API from the [todo list](https://docs.google.com/spreadsheet/ccc?key=0ApoVX4GOiXr-dGNKN1pObFh6ek1DR2FKUjBNZ1FmaEE#gid=0).

Should fit in style-wise, let me know if I missed something!