Flip out and replace shoulda with minitest

I don't really understand what version constraints to put in place to make shoulda run on Ruby 1.8 anymre, and we've mostly settled on preferring minitest for new code, so I think the easiest thing is to convert einhorn to using minitest.

@gdb mind taking a look? It's mostly find/replaces (and a couple grammar corrections), and the tests seem to pass for me now.