Added a cmd_sync method to WheelClient

This matches the behavior and semantics of the RunnerClient() method of
the same name.
Refactored WheelClient() to use sync/async mixin classes
Added function to execute WheelClient() functions asynchronously
Added cmd_sync, cmd_async methods to WheelClient docs
Renamed call_func to cmd for consistency; made compat alias

Added WheelClient() documentation on the non-eauth cmd & async funcs.