Set ServerName on crypto/tls Config

This fixes a bug introduced by Go 1.3 (http://golang.org/doc/go1.3#major_library_changes) which was actually a bug in 1.2.

This is not the best way to implement this but was the simplest thing I could do without breaking the API. Ideally, since this package is specific to APNS and APNS gateways are always the same, we should just be able to do something like `client := apns.NewProductionClient(cert, key)` and it will know what the gateway and server name is for production.