Autoprefixer Integration

[jescalan]

This PR will integrate autoprefixer into nib by default, and remove a lot of vendor maintenance stuff. Initial commit here just adds it to the pipeline, a lot of work still left to carefully strip the vendor-handling pieces from nib without breaking anything or removing functionality like the node-canvas stuff.

[notslang]

This is a nice start, but the tests are failing and we should gut out all the prefixing that nib does before we merge this. That way we're not prefixing twice, which could either cause performance degradation, errors, or both.

[jescalan]

Definitely, this is not intended as a full implementation, just getting the ball rolling. What are your thoughts on how to handle gradients? Autoprefixer has handling for gradients, but I know nib does image replacement as well...