Add support for assess

[DamienCassou]

I would like to add support for assess in buttercup. For example, a :buffer property could trigger the use of assess-with-temp-buffers. I would also like to see spec bodies be encapsulated inside assess-with-preserved-buffer-list. What do you think?

[jorgenschaefer]

There was some prior discussion about assess in #37. Basically, I'd be happy to use assess for buttercup instead of what buttercup uses internally. I'm not sure how this would work in reality, though. I have two reservations about it, though. First, I do not want a half solution – either buttercup depends on assess and uses it fully, or it does not depend on assess. Not something in between. And also, I would like buttercup to retain all the current functionality in the same way. I do not know if assess can provide that.