rework the Address / Company / Letter relationships

[Heinerion]

I have come to the conclusion, that:

1. A Letter should not be send to an address, but to a Client.
2. A Company should not have fields, which are related to the companies address, but an Address.

My suggestion are:

1. The implementation of a Client which is on the same level as a Company
2. The separation of Company and its address related properties in an Address property

[Heinerion]

inspired by Domain-Driven Design section 5