Refactor DifferentialChangesetParser -- pass 1 of N

Summary:
basically did my darnedest to pull out a TwoUp rendering view. Made a base class for the rendering views with "old" and "new" terminology rather than "left" and "right.

Future revisions will finish cleaning up the terminology within the DifferentialChangesetParser itself and more of the ideas within T2009.

Test Plan: been playing with differential all day

Reviewers: epriestley

Reviewed By: epriestley

CC: vrana, chad, aran, Korvin

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4117