Added method descendant_of

* R/commit.r (descendant_of): Added generic descendant_of and method
  for S4 class git_commit to check if a commit is descendant of
  another commit object.

* man/descendant_of-methods.Rd: Added documentation

* src/git2r_commit.c/h (descendant_of): Added C code to check if
  commit is descendant of another commit object in a repository.

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

* NAMESPACE: Updated