Adding support for Production Certificates

As described on the mailing list:

* an iOS variant has now the option to store two certificates:
  * Production 
  * Development

By default we will send messages to the ```Production``` server of APNs (assuming the actual certificate/passphrase are in place).

If the user includes ```"staging":"development"``` the message is send to Apple's Sandbox/Development APNs Server (assuming the actual certificate/passphrase are in place).