Fix 'Terminating since there is no workspace'

- This error used to prevent specs from being run on the iOS 6 simulator
  unless you used a tool like waxsim
- Fool the simulator into thinking a workspace is present by creating a
  message port named 'PurpleWorkspacePort' if it doesn't exist.
- Stub out -[UIWindow _createContext] because this throws an
  NSInternalConsistency error.