rpc.Server: deny requests basing on remote IP

Extends rpc.Server with Bind(...net.IP) and BindLocal() methods which
allow for whitelisting remote IP addresses.
gorilla/rpc: change import paths to x-formation/rpc
rpc.Server: appended authors note
rpc.Server: added tests for Bind() and BindLocal()
Merge pull request #1 from ppknap/bind-wip

rpc.Server: added tests for Bind() and BindLocal()