iOS8 Support

This is not yet ready to merge. For one thing, it needs to be thoroughly tested. For another thing, it depends on:

* https://github.com/appium/appium-uiauto/pull/45 (Building dynamic bootstrap into one file)
* https://github.com/appium/appium-instruments/pull/16 (Update sim killing logic to work for Xcode 6)

But I've had the basic testapp tests pass with this on iOS 8 so far.