Support for high-resolution sprites for checkbox/radio/alerts

* Grunt-recess updated to version 0.2.0
    * Updates LESS to version that supports min-resolution media query
* Added a form-hdpi.png to be loaded for high-resolution devices
    * PSD containing vector icons here: https://www.dropbox.com/s/s5bt6k2ekk2s5s4/imh-form.psd

Screenshot of new icons in use
![hdpi-form-icons](https://f.cloud.github.com/assets/1894775/1853064/0d930ac0-7706-11e3-88ca-c5c69944945b.png)