v0.20

        [DESIRED CAPABILITIES]
        - Add a desired_capabilities option that provides full control over the desiredCapability object
        - Add a new_from_caps constructor to distinguish against the normal new constructor
        - Add a ua option to allow injection of LWP::UserAgent for testing

        [MOO REWRITE]
        - Completed the Moo restructure (pr #103) with pr #122, fixing a bug in _check_ok (#121).