Added progress argument to clone method

* R/repository (clone): Added argument progress = TRUE to clone method
Added clone progress indicator

* src/git2r.c (clone): Added use of clone and checkout options.

* src/git2r.c (clone_progress): Added callback function to be called
  during the clone process to give progress feedback.
Updated documentation
Updated DESCRIPTION
Merge pull request #20 from ropensci/add-clone-text-progressbar

Add clone progress