Add C function git2r_checkout_branch

* src/git2r_checkout.c/h (git2r_checkout_branch): Add function to
  checkout branch

* src/git2r_checkout.c/h (git2r_checkout_branch,
  git2r_checkout_commit, git2r_checkout_tag, git2r_checkout_tree):
  Added argument who and msg to give information that will be used to
  populate the reflog entry
Add method checkout for signature git_branch.

* Add arguments who and msg to all checkout methods to populate the
  reflog entry