Allow setting the hostname, in case you are using a local relay.

We are using a local relay, as documented in https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion

This allows us to have great delivery performance, while still offering us all the magic tricks the Sendgrid SMTP API. Using the client library, we were unable to use our local relay because the hostname is hardcoded.

I've made the hostname configurable using a method 'setHostname()'. It defaults to the existing value (smtp.sendgrid.net), so this change be fully backwards compatible.