Added DsDbItem and DsDbTemplate for using deserialized data in tests

Check http://hermanussen.eu/sitecore/wordpress/2014/09/unit-testing-with-sitecore-fakedb-and-deserialized-data/ for more info.

I hope you like this approach. In any case; it's a separate project and doesn't require any changes to Sitecore.FakeDb as it is. Just an addition for those who want to use it.