Added parents method test case
Added method to get parents of commit

* R/commit.r (parents): Added generic parents and method for S4 class
  git_commit to get parents. Return list of S4 class git_commit.

* man/parents-methods.Rd: Added documentation

* src/git2r_commit.c/h (parents): Added C code to get parents of a
  commit.

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

* NAMESPACE: Updated
Fix parents method test case