Adds download panel for images

Closes #68 

* Adds download panel with dynamic size calculation and updated links
* Download panel button is disabled for gallery and horizontal view, and enabled for list view

![image](https://cloud.githubusercontent.com/assets/302258/4948681/b8ecd52c-65f8-11e4-84c9-03a51d36cbc2.png)
![image](https://cloud.githubusercontent.com/assets/302258/4948792/7000d0ea-65fb-11e4-9555-af9aade32184.png)
![image](https://cloud.githubusercontent.com/assets/302258/4948691/d71e14d4-65f8-11e4-811c-85790043592e.png)
![image](https://cloud.githubusercontent.com/assets/302258/4948692/e49762dc-65f8-11e4-8281-b6d20207ea70.png)


Note:
* File download sizes are not displayed because we have only jp2 size (not the derived jpeg size from jp2)
* We do not have support direct download links for thumb (= default) in digital-stacks. Currently, default link is disabled. 
* Few tests are pending for download panel
@jkeck Right. The plugin now has hooks to sul-embed codebase to communicate with download panel. Once done, I'll remove the download panel specific code and try creating a standalone plugin for IIIF images.