Create Input.MailMessage Class

[titarenko]

Create additional MailMessage class in Input namespace. It's role will be transferring of data from IEmailClient to another service, which will create Entities.MailMessage. Reason - Input.MailMessage represents Sender/Receivers using MailAddress and Entities.MailMessage will represent them as Person instances. Do necessary refactorings (IEmailClient, AeEmailClient, tests, maybe something else).

[titarenko]

Since I have received several questions related to this issue (guys! please write them here!), I'm putting here generic answer: "Actually this task is about simply adding new class to project. Don't bother yourself with additional work, I'll create further tasks related to these MailMessages later.".

[titarenko]

Please add documentation for class.