Added media query support to oldIE with respond.js

### Respond.js
@gavinballard wrote a [great article](http://gavinballard.com/using-respond.js-with-shopify-redux/) on using respond.js on Shopify. This is now implemented on TImber, seeing as Shopify theme store guidelines still dictate IE8 compatibility.
- This will work out of the box when themes are installed, so is invisible to merchants and nearly invisible to developers

### Minor ajax cart updates
- Server-relative image urls when the Handlebar.js template is used
- Minor adjustments to styles
- Likely to refactor the modal styles and JS shortly