Added blob method is.binary

* R/blob.r (is.binary): Added generic is.binary and method for S4 class
  git_blob to check if the blob is binary.

* src/git_repository.c (get_repository): Now works with a S4 class
  git_blob to get the reposiory from the path slot.

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

* src/git2r_blob.c/h (is_binary): Added function is_binary.

* tests/blob.R: Added test cases

* man/is.binary-methods.Rd: Added documentation

* Updated NAMESPACE