Separate EJApp into a standalone UIView + UIViewController

Hi, 
Just want to say amazing work, Ejecta is really impressive!

This pull request attempts to decouple Ejecta's ```EJApp``` into two components:
* A ```UIView``` subclass (```EJJavaScriptView```) that can load and display JS files
* A ```UIViewController``` (```EJAppViewController```) whose view is ```EJJavaScriptView``` (basically ```EJApp``` in it's current master state) 

I've only been able to test this pull request in the context of the example app (have not tested game center, device-motion control, audio) so it would be great to get some feedback.
I did try my best to maintain all backwards compatibility and continue to keep it easy for folks who just want to wrap up their JS file and go!

Our goal is to use Ejecta as a "viewport" embedded in any other ```UIView```. 
This pull request enables that functionality.

Ideally in the future we would also like to be able to spin up multiple instances of the ```EJJavaScriptView``` (essentially decoupling the singleton approach).

Additionally, would you be interested in moving the project to ARC?
```EJJavaScriptView``` is written with the ARC flag enabled. If you're absolutely against it I can move it back to manual memory management

thanks again for the excellent library!