Release 0.19.0

API changes:

- New Oid type
- Changed signature of Repository.create_reference
- Reference.oid and Reference.hex removed, use Reference.target instead
- Drop "del Index[path]" from the API, use Index.remove(path) instead
- Drop TreeEntry.to_object
- Changed signature of Repository.checkout
- Repository.create_blob_from_file removed, use instead new methods
  Repository.create_blob_fromworkdir and
  Repository.create_blob_from_disk

New features:

- Add len(TreeBuilder) and TreeBuilder.get
- Add Repository.merge_base
- Support changing the head with "Repository.head = refname"
- Improved support for diff
- Add support for clone
- Python 2: Support hex oids as byte strings
- Add Reference.get_object()
- Add Remote.save
- Add support for branches, new type Branch

Other:

- Upgraded to libgit2 0.19
- Partial documentation review

Thanks to Nico von Geyso, Daniel Rodríguez Troitiño, Bernardo Heynemann,
Rémi Duraffort, Andrey Devyatkin, Hervé Cauwelier, Jiunn Haur Lim, Richo
Healey, Carlos Martín Nieto, David Fischer, Fraser Tweedale, Jun Omae
and Xu Tao.