Add PytestTestEngine
List all DOM extension classes as extension classes for static analysis purposes

Summary: See D6895. We currenty whitelist one of the DOM classes, but should whitelist them all. Classes not in this whitelist raise warnings when users who don't have the corresponding extensions installed lint code. We don't want to require you to install every extension just to send diffs, so we whitelist stuff that we know would exist if you did install everything.

Test Plan: bwahaha

Reviewers: btrahan, Firehed

Reviewed By: Firehed

CC: aran

Differential Revision: https://secure.phabricator.com/D6896
Merge pull request #90 from dcramer/pytest

Add PytestTestEngine