use dashingjs as a module rather as a server

I wanted to use dashing to be part of our internal dashboard, but want to use it inside another nodejs server rather a separate server. These are the changes I made in order to make it work.

- add index.js that export `dashing.app` as a module
- add root to mincer paths for it to be look up correctly when dashing is a subfolder of the main projet
- add options to override some paths