[ALOY-842] Alloy: Have the measurement builtin use actual pixels for iOS as well

Right now, the measurement builtin converts dp to px and visa versa for Android only. For iOS, the given value is returned as is. This is understandable, since you would typically use the builtin's methods to get the right dimension for Android in it's system default px unit, based on a dp value you use on iOS. But of course this is not really accurate since on retina screens 1dp actually correspondents to 2px. So I'd propose have the builtin take this into account, either as the default or as an option. I prefer to have it as the default. The PR: https://github.com/appcelerator/alloy/pull/249 Related docs: http://docs.appcelerator.com/titanium/latest/#!/guide/Transitioning_to_the_New_UI_Layout_System-section-30088148_TransitioningtotheNewUILayoutSystem-UniversalUnitSupport