This commit adds basic git notes support to libgit2, namely:

* git_note_read
* git_note_message
* git_note_oid
* git_note_create
* git_note_remove

In the long run, we probably want to provide some convenience callback
mechanism for merging and moving (filter-branch) notes.

Signed-off-by: schu <schu-github@schulog.org>