Beginnings of a CSS utility belt: build on React's CSS value support, etc.

Building on issues #27 and #30. Happy to adjust based on how those feedback in those discussions.

A few things to note:

1. If we do end up including Autoprefixer support, we might want to create some sort of configuration settings that allow you to turn it off. I could see that being useful more generally, especially as we continue to add functionality.
2. I removed the `isValidValue` check in styleRuleConverter.js because the color objects returned by tinycolor (obviously) don't pass that test (yet are totally fine when cast to strings), and I didn't want users to have to manually type `.toString()` at the end of chaining. Open to suggestions here. Perhaps a better approach would be to cast to a string and then have a more intelligent, prop-based check for whether or not the value is valid.