Refactor MD5 code. Add unit test for FileFinder.
Summary: FileFinder::find() now uses PHP's md5_file() for all MD5 calculations and now has associated unit tests.
Test Plan: Ensure the unit tests pass.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5013