Added the ability to stop a feature if a timeout exceeded.

After updating to xcode 5 with iOS 7 instruments / automation it appears to have become a little unstable.
Sometimes a feature will launch but nothing will happen, instruments / simulator just hangs indefinitely.

This patch allows users to configure via config.yaml:
 - feature.timeout, which will kill a running feature if it exceeds this time. Defaults to 0.
 - feature.retry_attempts, number of times to retry a failing feature before giving up and moving on. Defaults to 1.