Added simplistic version check for test case skipping.

I wasn't happy with the renaming hack I had embarked on in #40, so I took a stab at this. @akhleung and I are going to talk about the existing plan for versioning in sass-spec, so if this doesn't fit into the plan I'll be happy to scrap it and go another direction.

It follows from discussion in #1. I thought I would make it as simple as I could, while still allowing a little flexibility.

Test cases can start with special comments like

    // VERSION=3.1.13,>3.2,<4.0.0

Where `VERSION` can take a list of version numbers that test should be run against, and operators > and < actually work like >= and <=.