Use specs2 instead of specs in all tests

This translates all tests to specs2's mutable API.  I've occasionally done a few funny things to avoid changing unfiltered-specs2, mostly because I'm not so familiar with specs2.

There are failures and errors, but they are exactly the same ones that occur in the basis revision, 1f121c8, in all `crossScalaVersions`, namely, `unfiltered-netty{,-uploads,-server}/test`.

I haven't touched the specs2 dependency versions, but maybe that's worth doing.

Fixes #30.