Added method to create stash

* R/stash.r (stash): Added generic stash and method for S4 class
  git_repository to create a stash in the repository.

* src/git2r_stash.c/h: Added C code to create stash in repository.

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

* NAMESPACE: Updated

* man/stash-methods.Rd: Added documentation