have safari native tap position 40px lower on the screen

fix for https://github.com/appium/appium/issues/2273 /
https://github.com/appium/appium/pull/2403 /

Conflicts:
	lib/devices/ios/ios.js
Merge pull request #2444 from imurchie/isaac-contexts-beta

Fix iOS context switch
Allow multiple calls to get contexts in iOS