Fix derived type being tracked as base type

[oscar54321]

Dear Joel,

#1 I took the liberty of making changes to address the issue #72 when collection item proxies created were of incorrect type. Please find PR below that addresses the issue.

#2 This is something I am not very fluent with. My gut tells me that the declaration below
public interface IChangeTrackable<T> : IChangeTrackable
should be changed to

public interface IChangeTrackable<out T> : IChangeTrackable (i.e. to to being contravariant).

This, however, causes conflicts with GetOriginalValue declaration.

TResult GetOriginalValue<TResult>(Expression<Func<T, TResult>> selector);
I am, unfortunately, not quite at arm lengths with contra- and co- variant interfaces and cannot clearly propose the best plant of action. Besides this strategical idea, my changes are quite limited and all the existing tests pass.