Issue #10: added methods needed to implement listshardpeer

[jrhea]

Added a listshardpeer method for ease of use:

$ ./sharding-p2p-poc -port=10000 -rpcport=13000 -client listshardpeer 2 3 
{"2":[PEER_1,PEER_2],"3":[PEER_2,PEER_3]}