Bug 925398: Refactor Gaia-ui-tests to use Marionette mixins, r=davehunt

https://bugzilla.mozilla.org/show_bug.cgi?id=925398 is introducing changes to marionette that allows us to easily use bits of the marionette runner and other classes for easy reusability, so we can mix and match parts we need to make other runners.

This patch pulls out the reusable parts of the gaia test runner into gaiatest/mixins/gaia.py, and modifies gaia_test.py and runtests.py to use the new mixins. The endurance and html reporting bits have been moved to Marionette.

This won't pass on travis since the marionette changes haven't landed.