Improve the testsuite

This adds several improvements to the testsuite:

- generic handling of ``UnsupportedDriverActionException`` so that it is not needed to catch it in each test anymore to mark them as skipped (I forgot it initially in #465 yesterday). this makes test more readable, and will also allow removing some skipping done in child classes in driver repos (before we started to skip them by catching the exception
- split MouseOver as not all of the actions are supported by each driver, and so you can test all or nothing. Selenium 1 and Selenium 2 drivers are both replacing the test by their own test doing only a subset of actions, thus not testing new actions when they implement it.
- added testing for all modifiers for keyboard events instead of testing only ``alt`` and no modifier