bdd mockito cleanup

> I'd love a PR with: 

>1. New feature highlighted on the main Mockito class (add an item to the bottom of the list, for example: "(new) Fluent BDD alias for verification"). 
2. Deprecate 'Then' class. Introduce 'Then' interface (you can create a new package org.mockito.bdd) that the 'Then' class can implement for now. 
3. Extra credit: also deprecate other concrete classes that leak from the top level BDDMockito 
4. Javadoc for deprecate class includes a deprecation reason 
5. Ensure new API that you have introduced contains @since information.

1. has already been solved in the original pull request.
I haven't introduced the Then interface yet. How are other interfaces going to be solved?
Thanks! Let's deprecate the entire class, too. In Mockito 2.0 we will make this class package-protected or move it to an explicit 'internal' package.