added timeout for the discovery of cells within a node 

Added a default timeout for discovering cells within a node. 

This was the stated issue https://github.com/celluloid/dcell/issues/64 .

Some things to review: 
- consistency of returning nil on timeout. I think it is, since when we try to find an unexistent cell within a node, it also returns nil. 
- SIGTERM handling. The process sends the terminate signal to the actors, but one of them hangs on some :zmqwait message. And then I have to kill. I thought timeout on the receive would discard the message, but apparently it doesn't. Or is this an issue that has been resolved already on celluloid master branch (future 0.16)?