Add support for Zoomify image base layer

I added this feature to address a local need (cc @jschell42), but thought it was worth seeing if you all would be interested. My  rationale for its inclusion is that it would be another, fairly easy route to work with a static, non-georeferenced image, with a major advantage over static images in that you have the speed of tiling. As I'm sure you're aware, for scanned historical maps, it frequently is the case that they don't respond well to georeferencing and are best handled outside of a web map environment. However, for detailed maps, the use of a single, static image can sometimes be pretty slow.

In the exhibit form, I added options for the Zoomify base URL, as well as the image width and height, which are included in the ImageProperties.xml file for any Zoomify-ed image. See the included image.

For a working example, see http://omeka.kevinrdyke.com/neatline/show/zoomify

Thanks!

![screen shot 2014-06-05 at 3 08 47 pm](https://cloud.githubusercontent.com/assets/666141/3193023/4c2a3d14-eced-11e3-9cfb-a1560adfed74.png)