Release v0.16.0

Changes since v0.15.1:

- Update to libgit2 v0.16.0
- Improve support for references
- New method Index.read_tree
- Rename Index.create_tree to Index.write_tree
- Fix compilation error with MSVC (issue libgit2#53)
- Fix a few segfaults (including issue libgit2#55)

Return bytes for paths (and encodings) on Python 2

Update setup file

Following PEP7 recommendations on function definition style.

Fix compilation warnings, remove unused variables

Optimize index iterator, refactor code