Makes it possible to use servePath to not prepend any directory names.

I had a problem where I didn't want to use any special directory names before my assets, for example:
`/assets/js/application.js` is not too nice, but `/js/application.js` is.
But the problem is that you couldn't set the servePath just to an empty string, because then it defaults to "assets".

By the way I don't really like my solution, because it seems a little bit hackish.

I'd really like to help out, because I like that connect-assets make the asset compiling and serving super easy. But there are some quirks...