RPCValidator completed for Pull Request

RPCValidator takes a String "call" and checks for the string's validity as an RPC. If the call is not valid, it builds and sends an error-response RPC to the sender. If valid, it pushes the call to the RPCHandler.
@euphzji 
Was "!params.has("port")" supposed to be "!params.has("sourcePort")"? If not what was going to go in the "port" field, need to pull in @ykravtsov to get that added to RPCBuilder.