Core SauceOnDemand functionality

This adds two dependencies: SeleniumFactory, and nose_xunitmp.

SeleniumFactory allows selenium drivers to be configured by a jenkins job via the Sauce OnDemand plugin. As a result, a lot of code in baseui.py related to configuring and creating a remote selenium driver is no longer necessary.

xunitmp is a workaround for an issue with xunit and running nosetests with multiple processes. It is effectively a drop-in replacement, but it is not used by default so it is still possible to use the existing xunit plugin for nose with no changes to configuration.