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)

@jchristo4 I was able to remove the `skip` line in https://github.com/sul-dlss/sul-embed/pull/164/files#diff-77196a95f172d1dd16dc482e816b6872L15 by moving the OSD Viewer instantiation last.  I'm guessing that means it was failing out in that plugin somewhere and wasn't executing any further js.

Let me know if you think this will be an issue.