receive *testing.T via an interface

this is related to:
https://github.com/stretchr/testify/issues/27

Receiving testing.T via an interface would allow [Ginkgo](https://github.com/onsi/ginkgo) to provide an implementation of the interface that works with Ginkgo's test runner.

I defined an interface in a new package called `interfaces`... if you don't like that there are other options (e.g. have an interface for this defined in each package)?  I'd be happy to iterate on this.

If this gets pulled in I'll add an adapter to Ginkgo (and some examples) to make using testify's assertion and mock libraries trivial.