Compatibility with pyScss 1.3, almost

This is what I came up with.  It turns out your `STATIC_URL` hack does still work.

Still not done:

* Trying to compile `/css/bar.scss` doesn't work, just because `Compiler.compile` doesn't consult the search path; it assumes you're feeding it a regular filesystem path.  This isn't terribly hard to fix; I'm just not sure what I want to do about it yet.  Maybe a new `Compiler` entry point?  But there are already a lot of those.

* The `DEBUG` toggle for only searching static files in prod mode was lost and hasn't been restored.

* I just remembered I didn't update `setup.py`, whoops!

Doing this has revealed some places that pyscss's extensibility is still pretty awkward.  I'm not in any hurry to go change everything again, but suffice to say I might have to send you another PR come pyscss 2.0.  API design is hard!  Still, this is rather a bit better than before.  :)