Create sfNoMailer.class.php with the same signature as sfMailer

This allow to avoid SwiftMailer initialization on all requests even if the application does not send any email.
Add NoMailer to the Changelog
Add a performance section with the no mailer part
Typo
Merge pull request #17 from damienalexandre/sfnomailer

Allow to disable SwiftMailer for real by using sfNoMailer