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 noted in https://github.com/FakeItEasy/FakeItEasy/pull/373#discussion_r19210367 I'd prefer making `IProxyGenerator` internal in a later PR.