updating API guide on wiki

[David Arthur]

FYI, I made a few updates to 
https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol

* Added offset commit/fetch APIs
* Updated error codes
* Updated RequestKeys
I noticed there are two APIs that are not yet documented: LeaderAndIsr 
and StopReplica. Also I'm not sure how to explain the 
StaleControllerEpochCode error code.

Cheers
-David