Finish setting up the Java Selenium functional test framework

Part of XWIKI-1346.

Goals:

• make it as easy as possible to use.
• make it low maintenance (ie tests shouldn't fail often and if some
  UI is changed it should be very easy to fix the tests)

Tasks:

• Continue improving our DSL for testing. Specifically add the
  ability to test over several skins and create one DSL per skin.
• Add DSL methods for asserting the rendered HTML