Include git2/odb_backend
Include git2/remote.h
Added line origins for Hunks in Pygit2.Patch
use chars like +,-,... for diff_delta_t in Patch.status
support for diffing the empty tree

added missing support for diffing tree-to-tree with NULL as argument.
(see issue #222)
Fix py_str_to_git_oid (closes #227)
Merge remote-tracking branch 'cholin/features/diff_tree_empty'
Merge remote-tracking branch 'cholin/features/diff_line_origin'
Merge remote-tracking branch 'richo/bugs/odb_stream'
Use git_{object,tree}_owner (#228)

Note that libgit2's git_commit_owner is missing from the public
interface, so we cannot use it.