Enable javascript server extensions to use sendMessageToBackend (and Drupal modules to receive these messages)

Hi thanks for this great module! In preparing for a drupalcamp demonstration I started hacking away at the module (drupalconlondon branch; i.e. with tokenChannels).

IMHO it would be very useful if server extensions could communicate back to Drupal modules via the server.js:sendMessageToBackend-function. This requires passing the reference to the SSJS-extensions (maybe full access server.js-module scope?) and an the introduction of an additional hook in the nodejs.module:nodejs_message_handler-function to be able to handle custom messageType in Drupal-contrib modules. A small patch attached.

I "created" a hook_nodejs_message_handlers()-hook - which works ok, but (architecturally) probably raises more questions than it solves (multiple modules replying w/ multiple responses etc.).