Add FEX_isUserVisible, iOS unit test target, Gemfile to Controls

Adds a `FEX_isUserVisible` category method on UIView, which optionally strengthens the predicate `FEX_isVisible` to only be user visible when the view in question is not completely obscured on screen by another view. `FEX_isUserVisible` is unit tested and has a example view in the Controls app. An iOS unit test target has been added to the project, which I hope will encourage greater test coverage going forwards. 

I also added a Gemfile to the Controls example, which points to the copy of the gem in the local repository. As long as you remember to `bundle`, you'll always be running Controls with the cloned version of Frank.

`FEX_isUserVisible` could be improved to check that the view is actually positioned on screen and not completely off of it.

Let me know what you think!