Add initial support for dismissing system alerts

* Links against the private UIAutomation framework to access
  UIAtarget, UIAElement, UIAAlert, and UIAApplication
* Only works on iOS 8.1
* Will fail if alert does not show up (eg. permission was already denied
  or accepted)
Add support for dissmising system alerts on iOS 7
Merge pull request #510 from joemasilotti/dismiss-system-alerts

Add support for dismissing system alerts