Added method tag to create tag

* R/tag.r (tag): Added generic tag and method for S4 class
  git_repository to tag "HEAD{commit}". Returns invisible new tag.

* src/git2r.c: Added "tag" to callMethods.

* src/git2r_tag.c/h (tag): Added function tag.

* tests/tag.R: Added test cases

* Updated NAMESPACE