Update IWebBrowser to incorporate all of the needs of Chorus

* Add ScrollLastElementIntoView to IWebBrowser
* Make DocumentText property behave the same in GeckoWebBrowser
  as it does in the Windows.Forms Browser
* Modify TestApp to allow for easier Gecko Testing
Address code review comments on Update IWebBrowser commit
Document a promise to reintroduce the DocumentText get
Merge pull request #37 from jasonleenaylor/ImproveGeckoFxWebBrowserAdapter

Update IWebBrowser to incorporate all of the needs of Chorus