Fix a couple of Ajax related path issues

  - Include context path when generating page specific JS
  - Include context path when making Ajax requests
  - Fix Ajax path matching (previously renderVersion was being set to
    "ajax")

fixes #1591 
What's the thinking on exposing this? I'm not necessarily opposed, mind you, just asking because I think it's a good idea to start consider parts of the JS side of Lift 3 “public API”, so we'll want to be a little more judicious about what is and isn't public.