Add cmd_sync and cmd_async methods to RunnerClient()
Document new cmd_sync, cmd_async RunnerClient methods

Also de-emphasize the direct master_call method. Since master_call is
inconsistent between RunnerClient and WheelClient we should instead
present a single, consistent interface.
Added two mixin classes, copied from RunnerClient, for executing funcs
Refactored RunnerClient to use sync/async mixin classes
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.