Added method to list stashes

* R/stash.r (stashes): Added generic stashes and method for S4 class
  git_repository to list all stashes in the repository as S4 class
  git_stash objects

* src/git2r_stash.c/h: Added C code to list stashes in repository as
  S4 class git_stash objects

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

* src/Makevars.in/win: Updated to include git2r_stash.o

* NAMESPACE: Updated

* man/stashes-methods.Rd: Added documentation