Removal strategy

Hi Ben,

I've added a "Removal" strategy in my fork on github. The idea is to explicitly remove (delete/destroy) specific objects at clean time.

I came upon the need for this when using Cucumber to test a remote PHP application where the application was using a separate database connection which could not benefit from the :transaction strategy and I needed the tests to run on an existing data set that would take a long time to import in the before step of every cucumber scenario.

If you get a chance, take a look and let me know what you think.

Cheers,
Matt.