Initial work with blame support
Fix Collate field in DESCRIPTION
Add C function git2r_blame_file

* src/git2r_blame.c/h: Add C function git2r_blame_file. Returns S4
  class git_blame.

* src/git2r.c: Add git2r_blame_file to callMethods
Add S4 class git_blame
Add S4 class git_blame_hunk
Add method blame
Merge branch 'add-blame-support'