Add a Grunt based solution for local development and deployment.

See README.md for more info.
Add html5shiv.js and respond.js for older browsers.
Clean up CSS.

* add missing vendor prefixed properties
* use hex colors when possible
* be more explicit with the background property
* use proper gradients for all browsers
* remove the unneeded img/header_bg.png
Move favicon.ico in root.
Switch to jquery fancybox for the screenshots.
Add image dimensions.