Support Safari Push and Mobile Device Management

Rounding out support different types push notifications:

* Title, Action and URLArgs are for Safari Push notifications.
* MDM is for Mobile Device Management.

This just allows these types of notifications through, it doesn't ensure that they are fully valid. I'm doing validations in advance.
Sorry on the delay on this.

This feels a little awkward – there is some implicit behavior that isn't clear from the API. It feels like it should only be the `if` and no `else`. Whether or not `"aps"` is included shouldn't _really_ depend on whether `"mdm"` exists.

My suggestion is:

```
if len(p.MDM) != 0 {
    p.customValues["mdm"] = p.MDM
}

p.customValues["aps"] = p.APS
```

And then including the approach for `APS` in this PR https://github.com/nathany/apns/pull/1 – it'll make the code much cleaner.