make fix

I fixed the Makefile so that you can easily just clone the repository and run `make` to build the minified versions. This is related to #35 but stripped down so it only includes the `Makefile` changes and not a buncha other nonsense with renaming files and whatnot.

I'm doing this so that its easier to [put this on cdnjs](https://github.com/cdnjs/cdnjs/pull/2619). bower is pretty awesome for collecting resources for websites, but I personally tend to use CDNs as we move to production and think this would be useful. Speaking of cdnjs, I hope you're cool with the [package.json](https://github.com/deanmalmgren/cdnjs/commit/c318f19e38c74236ef48fdec2604c1cb7b188c4c) I put together for d3-tip on cdnjs. I didn't include your email out of respect for some reasonable amount of privacy...