Signature
=========
- Implement equality interface
- In Repository's create_commit/create_tag check signatures encoding is right

References
==========
- Wrap missing functions: git_reference_foreach, git_reference_is_packed.
- Write more twisted tests. Like accessing a reference deleted by someone
  else.

Other
=========
- Make the Py_LOCAL_INLINE macro to work with Python 2.6, 2.7 and 3.1
- Use surrogateescape in Python 3, see PEP-383
- Expose the ODB (Repository.odb)