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).
Technically a breaking change, since this is a public constructor for a public class.
However, while clients probably enjoy using `FakeManager`s that they get from FakeItEasy, I don't see any need for them to construct them.

The method could probably be made internal in the future (that is, not in this PR).