Sending notifications to multiple registration ids

[vairix-ssierra]

As suggested in #16 this commit implements "broadcasting" to multiple devices.

[twigbranch]

Couple things:

1. I read that there's a 1000 device limit on registration_ids- this commit doesn't take that into account does it?

2. Wondering if like apn_on_rails there should be a group_notification, so instead of storing each notification to each individual device, the notification is stored once for a group of device ids?

[vairix-ssierra]

@twigbranch , I can't find the docs where the 1000 device limit is mentioned ... can you provide a link to it so I can fix this?

I'll take a look to the apn_on_rails approach about creating group_notification and see if we can use it here.