Link to image in webclient

Adds support for linking to image(s) in webclient (see below).
https://trello.com/c/60tfGnF4/3-link-to-image-in-webclient

Also fixes bug when creating PDF with someone else's images:
https://trello.com/c/RFTL1649/48-cannot-create-pdf-on-nightshade

![screen shot 2014-05-09 at 14 53 53](https://cloud.githubusercontent.com/assets/900055/2928766/68f9498e-d781-11e3-8359-d4f175fb7ca6.png)