Add WordPress.com OAuth2 plugin

Summary:
This plugin provides an OAuth authentication provider to authenticate users using WordPress.com Connect.

This diff corresponds to the github pull request: https://github.com/facebook/libphutil/pull/36

Test Plan: No test plan, at the moment.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D9004