Refactoring: Refresh testify's implementation of assert tests

[eduncan911]

It's time to refresh the testify implementation of our assert package. I originally chose to embed it into the mspec package for several reasons:

• To remove dependencies from the code.
• The author said that testify wasn't stable yet and could change at that time.
• I didn't want to increase complexity with package management, just to run tests.
• Some assert tests could not be ported as-is.
• Needed to override their error handling.

That last one was the key reasons. I could have mocked up a fake context to override their version of testing.T, but that felt like a lot of bloat given the other concerns above. So in the end, I just included a slightly modified version with mspec.

Since then, I've found a few small bugs and even a broken test or two with that old testify version. I've also noticed additional comparisons that testify now has.

So, it's time to refresh this package with their latest. And also, review the license to see if it has changed.