Initial work with note support
Fix Collate field in DESCRIPTION
Add S4 class git_note
Add method note_list
Rename class S4 git_note slot note to message
Add note_list test case
Add slot annotated to S4 class git_note
Add default argument ref = NULL to method note_list
Add C function git2r_note_defualt_ref

* src/git2r_note.c/h (git2r_note_default_ref): Get the default notes
  reference for a repository

* src/git2r.c: Add git2r_note_default_ref to callMethods
Add note_default_ref generic/git_repository method

* R/note.r (note_default_ref): Get the default notes reference for a
  repository

* man/note_default_ref-methods.Rd: Add documentation

* NAMESPACE: Updated
Add slot refname to S4 class git_note

* R/note.r: Add character slot refname to S4 class git_note to hold
  name of the reference
Add C function git2r_note_list

* src/git2r_note.c/h: Add function git2r_note_list and helper
  functions to list notes in repository

* src/git2r.c: Add git2r_note_list to callMethods
Update documentation
Add test case check notes default ref
Add C function git2r_note_create

* src/git2r_note_create.c/h (git2r_note_create): Add function to add a
  note to a commit.

* git2r.c: Add git2r_note_create to callMethods
Add C function to check S4 class git_note argument

* src/git2r_error.c (git2r_error_check_note_arg): Check S4 class git_note
Add C function git2r_note_remove

* src/git2r_note.c/h (git2r_note_remove): Remove the note for an
  object

* src/git2r.c: Add git2r_note_remove to callMethods
Add note_create generic and git_commit method

* R/note.r (note_create): Create note for a commit

* man/note_create-methods.Rd: Add documentation

* NAMESPACE: Updated
Add more note test cases

* tests/note.R: More test cases
Fix git2r_note_remove to use annotated oid