Refactor log writer

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.
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 we allow others to use the mail component more easily outside of Laravel
Fix some docblock comments and namespace imports