add GetCurrentOffset to client

For review @eapache @wvanbergen 

@wvanbergen, I used your code (nearly verbatim) from #89 
Likewise, this default can be moved out of the switch, leaving it with only one case - I believe there is a way to do a single-type switch more simply, but I don't remember off the top of my head. `err, ok := err.(KError)` or something?