Fixes #31

I added support for posting to an individual user using the following api call:
/event/EVENT_NAME/subscribers/SUBSCRIBER_ID.

e.g.:
curl -X POST http://localhost:9080/event/test/subscribers/QLz_kYGtRxQ -d "msg=test"
where test is the name of the event and QLz_kYGtRxQ is the id of the subscriber.

This way you can target all subscribers for an event or target a specific subscriber.
This is especially useful if you store the user->device mapping in your own back-end. Most users will use this scenario because this back-end server will send the event's to the pushd server.