Add some basic signature stripping

Summary: See discussion in T789. Covered the obvious cases, at least. We can refine this as we get a larger sample size.

Test Plan: Unit test coverage.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T789

Differential Revision: https://secure.phabricator.com/D2154