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).
As written, `EnsureInitialized` is just `Fetch` with a suppressed return value (except that `Fetch` verifies that the proxy hasn't changed, an action that I think could be moved to `EnsureInitialized`). Could we collapse the methods?