Test suite setup

A simple continuous testing setup using gulp, [testem](/airportyh/testem) and [qunit](http://qunitjs.com/). 

These are by no means fixed testing dependencies, just what I'm most familiar with. Feel free to swap these out.

I've added the first test for bar charts in `tests/charts/bar_test.js` as an example of how to proceed.

You'll need to run `npm install` and install the [PhantomJS](http://phantomjs.org/download.html) headless browser using your favourite package manager, then you can just run `gulp test` to get going.