Host configuration fallback

Host configuration is now optional. If not given it will generate it automatically from the request context configuration.

For more info on request context: [How to generate URLs from the Console](http://symfony.com/doc/current/cookbook/console/sending_emails.html)

**Symfony 2.0 support dropped as request context configuration is only available in Symfony 2.1+ (Composer file updated accordingly)**

I've also added an example nginx configuration in case the server allows only certain php files to be processed by fpm (Which is the case for most nginx installations I guess). It might come in handy.