Fxapom

* Add Python 3.6 classifier

* Remove support for the Marionette client

* The latest versions of Selenium and Firefox use Marionette through
GeckoDriver.

* Migrate page objects to PyPOM

* Wait for the sign in page to load when not in a popup

* Pulled in PyFxA 0.2.0, to fix mock-import issue

* Pulled in latest pyfxa 0.1.3

* Fix regressions caused by removing implicit waits and Marionette functionality