ios touch move fix

DO NOT MERGE YET!

see https://github.com/appium/appium/issues/3021

The aim is to make stuff `touch.moveTo(null, x, y)` use an absolute position. This is what is happening on Android (see [here](https://github.com/appium/appium/blob/master/lib/devices/android/bootstrap/src/io/appium/android/bootstrap/handler/TouchEvent.java#L77)). 

I don't think the behavior was properly specified in the specs, but @Jonahss javadoc [here] (http://appium.github.io/java-client/io/appium/java_client/TouchAction.html#moveTo(org.openqa.selenium.WebElement,%20int,%20int)) mention `absolute position`. (not only for  moveTo, but also for press)

@imurchie @jlipps you think it is correct behavior/ correct fix? 
 
Do we need to restrict to specific actions?