Link to HTML versions of lesson plans in unplugged activities

- Add PDF download button for unplugged activities

# Before

![image](https://cloud.githubusercontent.com/assets/206973/3726954/b68f8d24-1698-11e4-861e-6b50c57db255.png)

# After

![image](https://cloud.githubusercontent.com/assets/206973/3726952/abd1cb18-1698-11e4-8e49-cd2a29ed2f5e.png)