Updates to footer contents

Closes #205 

* Removed `http://` from PURL link
* Shortened PURL link text to druid for embed widths smaller than 400px
* Removed `No image selected` text in download button for embed widths smaller than 300px 

![image](https://cloud.githubusercontent.com/assets/302258/5273030/d554c0b4-7a37-11e4-8bbf-f5b7ac37ee34.png)
![image](https://cloud.githubusercontent.com/assets/302258/5273037/e1891ace-7a37-11e4-94d3-93d58d3e04f8.png)