Model refactoring

[alexeyshockov]

I suggest do some model refactoring.

By steps:

1. Introduce MailAgentInterface and default ezcomponents/mail implementation. This interface will contain all actions on mails (sending, removing, receiving). This interface represents mail agent from a real life, like mutt and Outlook.
2. Refactor Mailbox and Mail classes to be abstract data containers (Mailbox will be simple collection of Mail objects with some specific methods).

[fureev]

1 done

[alexeyshockov]

Excellent :)

I added another related issue — #6 — for extracting mail client implementation.