Move the interface closer to the actual content of GCM notifications

Replace `collapseKey` with `collapse_key`, etc.

It would also be really cool if we could make it possible to send pre-built messages, for when the user knows what she is doing:

``` js
var gcm = require("gcm");
var sender = new gcm.Sender("<api key>");
sender.send({
    data: {
        hello: "world"
    }
}, "<registration id>", callback);
```
