Add `subscription` callback to  PrivatePub.sign

If options given to `sign` include a `subscription` function, it
will be called with the newly created subscription object. This
way users can add a Faye callback to know when a subscription is
ready or cancel a subscription in the future.
Expose methods for unsubscribing and getting a channel subscription object.

Store subscription objects on an internal object and expose
three new methods: `subscription(channel)`, `unsubscribe(channel)` and
`unsubscribeAll()`
Add specs for options.subscription callback and unsubcribe methods.
Merge pull request #60 from vic/master

private_pub.js subscription callback and unsubcribe function.