APN: Need some help / more detailed sample, pls

@raix thanks heaps for this package.

This is my first time trying to get APN up and running. Rushing to get beta running with TestFlight before the weekend ends and Apple shut it down till the 29 Dec.

Anyway, wondering if you have an example or some additional code snippets, please?

For example, is "pushId" meant to be something like my app identifier
I was not sure what the webServiceUrl is too?

```
apn: {
pushId: 'com.push.server'
webServiceUrl: 'http://some.server.com'
},
```

I have put a push.js file in my server directory with your Push.init code:

```
Push.init({
/*gcm: {
pushId: 'xxxxxxxxx'
},*/
apn: { // setting this on client throws security error
passphrase: 'my passphrase',
// pem files are placed in the app private folder
certData: Assets.getText('certfile.pem'),
keyData: Assets.getText('keyfile.pem'),
production: false, // or...
gateway: 'gateway.push.apple.com',
}
});
```

I am seeing the following in the console:
WARNING: Push APN is configured to production mode - but server is running from localhost

Anyway a "hello APN" type of example would be awesome :-)

thanks again, looking forward to getting this up and running.

Start with the push.js Push.init example and the APN configuration shown in the issue. Clarify the intended meanings of pushId, webServiceUrl, certificate settings, and production mode, including the localhost warning. Done means a concise “hello APN” example addresses these setup questions.

Written by the indexing model from the issue text.