Move Utilities#get_repository into Repository#find

In an effort to start moving all repository details moved
into their own respective classes and objects for a better
design model and testing, move the repository reference into
a thread-safe object for later re-use.
Small refactoring to simplify
Rename Repostory to Repo for brevity
Merge branch 'master' into repository-class

Conflicts:
	lib/git_statistics/collector.rb
Merge pull request #52 from kevinjalbert/repository-class

Move Utilities#get_repository into Repo#find