Added Ajax management for GET requests

- Created methods in "gotit.js" for Ajax system purpose
  -> GET requests to do a partial update of a web page
  -> POST requests should be implemented soon
- Corrected the module pattern of Javascript files
  -> No more "gotItXXXModule" in the window object, only "window.GotIt.XXX"
- Adapted some views according to the new CSS files
- Fixed a bug for the CSS BorderLayout making nested BorderLayout not
  possible
- Added some images for the application menu
- Changed "sign-in" and "sign-up" views logic: they are now managed by the
  StaticController rather than the UserController
  -> This means their URL is now "/sign-[in|up]" rather than
     "/users/sign-[in|up]"
  -> Adapted SecurityConfigurer for the new URLs (sign-in and out POST)
Started implementation of additional services

- Creation of assessments to students in courses
- Changed some standard views to fit the new CSS files
- Will now finish the virtual desktop implementation