Updates to Payment Apps wiki page

Hi Ian and WG,

I have made some updates to the payment apps wiki as requested:


   - I added a comment about us needing to clarify our scope. I think a lot
   of what was in there assumed that the scope was only payment apps that run
   in a browser and ignores the possibility of other apps that can still be
   interfaced from a browser over the Web.
   - I added a few definitions to differentiate between browser-based and
   web-based apps (per scoping question above).
   - I also dropped the hybrid and web-technology apps as I'm not sure
   these have any impact on integration with a browser (i.e. an app either
   runs in the browser or doesn't which is all that matters from the
   perspective of integration with the browser).
   - I added the following line to the display text:
   "The browser should display matched payment apps in an order that
   corresponds with the order of supported payment methods supplied by the
   payee"
   - I made changes to the advantages and disadvantages of the invoking and
   response sections based on discussions with Ian.
   (Although I'm still not sure I agree that JavaScript encapsulation has
   an advantage of flexibility, I think it's the opposite)
   - I corrected the steps in the JavaScript encapsulation approach to
   include passing the response to the browser.
   - Added a hybrid response approach
   - Made some minor changes to the data collection points