Make download optional through URL parameters and add to Embed panel.

#### Embed panel
![embed-this-download](https://cloud.githubusercontent.com/assets/96776/4959812/69df0972-66bc-11e4-8df1-91a67bcf431c.png)

#### Sandbox
![hidden-download](https://cloud.githubusercontent.com/assets/96776/4959811/69c5cf66-66bc-11e4-856d-de28b77095c4.png)