add GetCurrentOffset to client

For review @eapache @wvanbergen 

@wvanbergen, I used your code (nearly verbatim) from #89 
to get it in a separate request, pass in a different value to the `where` parameter.  To get it in the same request, I don't know if kafka lets you spec. multiple blocks with the same topic/partition. Sarama doesn't (it's a map underneath) but if Kafka does and this is useful then Sarama needs to move to slices instead of maps.