SimControl can erase individual Xcode 6 simulators

Required to fix "RESET_BETWEEN_SCENARIOS is broken for Xcode 6+" https://github.com/calabash/calabash-ios/issues/518

Allows an Xcode 6 simulator UDID to be passed to RunLoop::SimControl::reset_content_and_settings.

```
sim_control = RunLoop::SimControl.new
sim_control.reset_content_and_settings({:sim_udid => 'A81E88A0-770F-4125-9CB7-9D9E0DCE4509'})
```