Autoprefixer support and fixed tests

I was missing a sass package for Meteor that also ran Autoprefixer, so I added it as an option (and dependency). It's now possible to enable "autoprefixing" by setting enableAutoprefixer to true in the scss.json config file. Ideal would be to have this as a separate dependency (e.g. a postprocess step in the Meteor isopack process), but it seems it isn't possible (yet).

It looked like the tests needed some tiny bit of love as well, so I fixed them and added a test for checking whether autoprefix ran or not.