The Ambient Split

Took "some" of Yevhen Bobrov's advice to heart :-)
There is no global UnitOfWork, so ditched the static methods as they
didn't make sense.
Repository and AmbientUnitOfWorkAwareRepository split ways as to not
confuse people using either. Using your IOC or a more DIY approach are
still preferred these days over ambient access because it's more magic
and well, let's face it more dangerous to leave things dangling.
Made the lookup of the ambient UoW internal so you don't get any ideas
about using it for other purposes.
Yes, the UnitOfWorkScope is still [ThreadStatic].
Yes, I'll explore more pluggable ways of having your ambient cake and
eating it too.