Support for launching retina and tall devices


I made a few changes which impact the public API used to launch the simulator:

* Direct launching goes through launch_ios_app as before, but with changed parameters: an app_path and an options hash. 
  
    This also cleans up the code to parse a http request into a call to launch_ios_app.
* HTTP launching passes the device as a parameter to a new launch_ios_app endpoint, replacing the seperate ipad and iphone endpoints.
* I removed the launch_ipad_app and launch_iphone_app methods which don't pull their weight anymore and aren't used by Frank. 

I'll be submitting a pull request to Frank shortly which leverages this support.

I'd love to hear your thoughts on this.