Fix issue #8 - move IoC setup to BeforeEachExample

Fix for issue #8.

The problem was that `InitializeTest`, where the generic Spec class sets up its DI container, was not being called until after `BeforeEachExample`. Moving the setup code out of InitializeTest and into a BeforeEachExample override fixed the problem.

I decided to remove `InitializeTest` entirely. It seems to be redundant now that we have `BeforeEachExample`. This is technically a breaking change.