Implement rewind backend
Merge branch 'feature-rewind'

Conflicts:
	lib/twit.rb
	spec/twit/repo_spec.rb
	spec/twit_spec.rb

Removed outdated code for Twit.include

Code conflicts:
        lib/twit/repo.rb

Rewind previously used Twit.open as an alias for git checkout, but
Twit.open now only works on branches. Instead, simply set the HEAD
directly to the new commit.