adds enhancements to rotation

* replaces `@current_rotation` with two functions:
      - `device_orientation` => current UIDevice orientation
      - `status_bar_orientation` => UIApplication statusBarOrientation
* updates `rotate(dir)` to use `device_orientation`
* added `rotate_home_button_to` function
  - `rotate_home_button_to {down | up | left | right}`

KNOWN ISSUES:

1. device_orientation is always "unknown" on the simulator until a
   rotation occurs
2. `rotate_home_button_to` can leave the `device_orientation` in an
   inconsistent state if a playback fails to rotate - happens if the
   view controller does not respond to rotations

https://github.com/calabash/calabash-ios/issues/156
https://github.com/calabash/calabash-ios/issues/72