Add image preview

Closes #27 

* Added image/record preview independent of filmstrip that can be added to any image or list of images
* Preview jQuery plugin automatically adds preview buttons (small icon and "Preview" block) and links to the source images 
* When preview is invoked from within a filmstrip, preview disables scrolling temporarily and enables when closed
* A filmstrip can be rendered with/without previews 

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![searchworks-issue27-1](https://cloud.githubusercontent.com/assets/302258/3233440/463eac44-f0be-11e3-8d5d-7145b4f3ff18.png)

@jkeck  Removed `root_url`. Now preview content fetch calls use relative path instead.