Dynamically load geckofx from whatever version is present

We don't want to tie every application that uses this
assembly to the same gecokfx version.
Cleanup GeckoFxWebBrowserAdapter

* some code cleanup and a new user displayed error message
  in response to review.
Merge pull request #34 from jasonleenaylor/UseGeckoReflectively

Dynamically load geckofx from whatever version is present