Don't use session to track which page we show

Why?

 - It breaks the back button
 - It's a PITA for @demwunz and I to work with when we want a specific
   page.

The page to show is worked out by the url or from the referring page
in the case of a PUT request.

Changes:
  - Removes ClaimSessionTransitionManager
  - Adds ClaimTransitionManager
  - Adds TransitionManager, an abstract class with a cool DSL for
    describing transitions
  - Cleans odd spec formatting
Merge pull request #11 from ministryofjustice/track-form-state-in-url-not-session

Don't use session to track which page we show