Fest

Fixed bug in Tribe (Wordpress) event deletion
Use `google-auth` library for authenticating with Google Calendar
Drop support for Python 2 and 3.3

* Removed hack-y code to add custom fields to Wordpress post objects -- users can add this themselves if necessary

* Allow `GoogleCalendar.get_events()` to receive keyword args

* Add `google-service-http` pip to help get `http` object for Google resource API

* Updated Graph API interface to require page access token. Details [here](https://newsroom.fb.com/news/2018/04/restricting-data-access/)
* Removed Slack interface

* Refined Slack interface