WordPress.com OAuth2 plugin

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

This diff corresponds to github pull request https://github.com/facebook/phabricator/pull/593/ and had its libphutil counterpart reviewed in D9004.

Test Plan: Configured WordPress.com as an authentication provider, saw it show up on the login screen, registered a new account, got expected defaults for my username/name/email/profile picture.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9019