notification: change `Notification#Data` to a map of interfaces to
support json objects for GCM. Send objects as:

```
?service=foo&subscribed=santa&someobj[prop1]=foo&someobj[prop2]=bar&val=meh
```

To generate

```
{
  "val": "meh",
  "someobj": {
    "prop1": "foo",
    "prop2": "bar"
  }
}
```
fmt: updating imports to fork