Add support for HDPI form images

* The images from form.png have been recreated as vectors in a PSD
    * PSD file is available here: https://dl.dropboxusercontent.com/u/7254322/imh-form.psd
* High-DPI devices will load the hdpi icons instead of the standard icons

Screenshot of updated icons:
![hdpi-form-icons](https://f.cloud.github.com/assets/1894775/1847198/258819ec-763c-11e3-9af8-26a3eb3ad6b9.png)