Started to improve the visual test project.

The view controllers are now in storyboards so we can easily add more.
Fixed a bug with non-retina default back button on iOS7. Previously it just used the retina image for the back button for both, which didn't look right.
Added a renderer precedence view controller.
Fixed a bug with the renderer not taking precedence over the theme. Added a visual test for that too.
Merge pull request #129 from ChrisGrant/master

Fix for renderer precendence