Re-factor StyleGuide for ease of type addition

This change makes it easier to understand `StyleGuide::Base` inheritance. The
template method pattern makes it easy to scale style guides to scale
horizontally: just add the new language style guide and inherit off of `Base`.

Changes:
* add `Violations` object to manage unique violations
* refer to modified lines as changed to make it easier to reason about
* introduce `UnchangedLine` class
Merge pull request #1 from thoughtbot/master

Re-factor StyleGuide for ease of type addition