Remove unused import statement
Suggest Guzzle for Mandrill and Mailgun mail drivers

This way we can allow developers to use their own logging classes and provide an easy way for them to use them with the mail component.
Move transport registration to separate class

By moving this functionality outside of the service provider and into a class that extends the Manager class, we offer an easier way to user to register their own mail drivers.
Merge branch 'feature/improve-mail-component' of https://github.com/driesvints/framework into driesvints-feature/improve-mail-component
Clean up a few small things.