add TunnelBroker::Client class

This class is what is used for communicating with the TunnelBroker API.

Not much documentation here yet...
Merge pull request #13 from theckman/add_client_class

add TunnelBroker::Client class
Merge branch 'master' into improve_messenger_class
Merge branch 'improve_messenger_class' of github.com:theckman/tunnelbroker into improve_messenger_class