2.0.0: Total redesign (simplifying) of mail interface.

Because this interface is being used a lot we found some flaws/short-comings in how it was designed.

For example:
* We always use it to send html mails and never plain text mails.
* The exceptions are never used/thrown because in the implementing classes we mostly rely on php's build-in exceptions like RuntimeException.
* It's damn annoying to write implementations for the MailAddress MailSubject etc classes because they are always the same, also in the general sense of things a string will do which can be validated in implementation if needed. The same rule applies on the Attachment class, so we added it here.