Refactoring of user interface for diff print functionality

* Moved diff print functionality from diff to diff_print method.

* src/diff.r (diff): Removed argument filename. Always return S4 class
  git_diff object.

* src/diff.r (diff_print): Add method to print diff to either a
  character vector or a file.

* src/git2r_arg.c, src/git2r_error.c: Updated argument check of valid
  filename argument to diff functions

* Update NAMESPACE and documentation