Rough and ready cut over of btcctl to use the btcjson command api.
Add missing return; avoids panic.
Switch rpcserver.go over to using the jsoncmd apis.
Alphabetise command handlers.
Split the massive switch statement in rpcserver into a map of handlers.

Cleans up a the code a lot and removes the need to be careful about
shadowing err.

Much rejoicing from jcv, jrick and davec.