Add support for image dropdown fields (WIP-ish)

* Add support for auto-sizing of `field_image`s
* Add image-only dropdown field type
* Constantize some things in core
* Extract a few related methods in core

Screencast: http://screencast.com/t/HZXrH27aH5

![image](https://cloud.githubusercontent.com/assets/206973/2824404/2e21233e-cf31-11e3-92d0-3e5fd67b7334.png)

![image](https://cloud.githubusercontent.com/assets/206973/2824411/422772de-cf31-11e3-8cdb-5f3a0db31f40.png)

Still needs further testing and a bit of CSS styling tweaks on the dropdown.