Moved fake manager creation and initial configuration into the proxy generator (Part I)

This is the first part of PR #372 (we split it up into 2 PRs as mentioned there).

It solves #371 partially, namely that the default behavior is available inside the c'tor call. Therefore it also solves the bug #368 (see the `when_faking_a_class_which_calls_virtual_property_members_in_constructor` test case).
This is a public class, so in theory a breaking change. I can’t imagine why anyone would make one of these, though. It’s possible that this and `IProxyGenerator` should be internal. For now, I’m not so worried about this.