Generate a source map!

This required passing more "raw" information to uglifyjs than we'd been doing previously. No more catting all our sources together.

See [this blog post](http://jstarrdewar.com/blog/2013/02/28/use-uglify-to-automatically-strip-debug-messages-from-your-javascript/) for more info on conditional compilation with UglifyJS.

The upshot of this is that it's possible to source `dygraph-combined.js` but debug as though you'd sourced `dygraph-dev.js`.

Fixes #420