Minify fix (+index.js -> d3-min.js rename?)

The uglifyjs task in the Makefile was broken so I fixed that.

But as I was doing this I realized that the original file must have been called `src/d3-tip.js` instead of `index.js`. I don't see any reason to put the file in a `src/` directory, but I admittedly think its a little more obvious which file to edit when its called `d3-tip.js`. At the risk of being forward, I took the liberty of renaming `index.js` -> `d3-tip.js` in the same PR --- bad form, I know.

If you prefer `index.js` I'm happy to modify the PR to retain the index.js filename or you can tell me to bugger off and mind my own damn file naming conventions. haha