add index.js for node require, fix browser compatibility, add code coverage, update readme

Added an index.js that references the src/daterange.js so node users can just 'var daterange = require('daterange');' after they have npm installed the module.

Updated the reference to 'exports' within the daterange class to use 'daterange' instead. This fixes in browser compatibility issue because browsers don't have an exports reference.

Added code coverage with [istanbul](http://gotwarlost.github.io/istanbul/).

Updated the readme with a few more examples and fixed typos.
HA! thank you!