new package idea: assert

[dominicbarnes]

I've always really liked the github.com/stretchr/testify/assert package (and the related require package), but I think there's room to improve upon writing test assertions:

• adding color-coding to the messaging (maybe?)
• adding better support for types like time.Time and time.Duration
• cleaning up the method naming
• better diffs for large or complex values

I'll probably write up a "spec" document just to see if there's enough I would change to actually warrant a new package or try to contribute upstream to testify.

[dominicbarnes]

If I did write an assertion library, would be nice to have a play on the name "got" as it is commonly used in test vernacular.