Remove require statements from repository.rb

Put them into the gitdocs.rb with the other require statements
Refactor change count to use Rugged

* added Gitdocs::Repository#author_count which will return a name of authors
  and counts of their commits
* refactor the runner to use this new method (changing the name of the main
  method to #latest_author_count
* also refactored the code around the change listing to be more DRY