Initial work on pull functionality

* R/pull.r (pull): Added file with generic method 'pull' and method
  for a S4 class git_repository

* man/pull-methods.Rd: Added file

* Updated NAMESPACE and DESCRIPTION
Initial work on fetch functionality
Added argument name to fetch method

* R/fetch.r (fetch): Added argument name to give the remote's name to
  fetch.

* man/fetch-methods.Rd: Added documentation

* Updated NAMESPACE and DESCRIPTION
Added code to fetch remote

* src/git2r.c (fetch): Updated function to fetch remote
Merged ongoing work with pull functionality into master branch