Added interface for models, which can be posted to ChurchTools API

[stollr]

This gives us more type safety in the update method of the request builder and in the create method, that I am planning to implement soon ;-)

[DumbergerL]

@Naitsirch This PR is a great idea! Only a few suggestions:

• I moved the Interface to a separate folder similar to the traits
• i renamed the interface. "PostableModelInterface" seemed for me a bit technical. I know you ment the Post-HTTP-Method but i would keep this in a business/data related language. Are you fine with "UpdateableModel"?

[stollr]

I also thought about something like "updateable" but I had doubts because it is a bit confusing, because the models theirselfes are always updateable by their setters.

But naming things is one of the hardest tasks in Software development :-D

Anyways, I am alright with your changes ;-)