Adds initial version of IIIF OpenSeadragon image viewer plugin

Closes #11, #65, #92

Test object - http://purl.stanford.edu/cn006dx2288

![image](https://cloud.githubusercontent.com/assets/302258/4890703/50ed5860-63a1-11e4-975f-05cdae5263c4.png)
![image](https://cloud.githubusercontent.com/assets/302258/4890710/5bb97c74-63a1-11e4-87da-a1ac52ef1a78.png)
![image](https://cloud.githubusercontent.com/assets/302258/4890712/64d3c51c-63a1-11e4-9e20-f923cc7c8bf2.png)
![image](https://cloud.githubusercontent.com/assets/302258/4890720/76b26d56-63a1-11e4-9c0e-77b4352a0807.png)

Pending features in IIIF viewer plugin : 
- [ ] lazy loading in all views
- [ ] links between views
- [ ] remove thumbs container for single image (in list view)

I'll create separate tickets for pending features. 

We shouldn't have the empty `()` in the method definition if it doesn't take any parameters.