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