Recover from unresponsive instruments process

Register for kCGSNotificationAppUnresponsive and if the process is Instruments then terminate it.
This enables Appium to continue even when instruments is unresponsive. The current behavior is to wait forever.

To test this feature works properly, run ./instruments.sh from https://github.com/appium/ForceQuitUnresponsiveApps

instruments.sh will launch a process that's unresponsive and appium will force quit the process. The unresponsive process can be visually confirmed by using OS X Activity Monitor.