Add progress indication to document preview loading

Suggested by @bp323 in comment https://github.com/oaeproject/3akai-ux/pull/3336#issuecomment-37633956

![screen shot 2014-03-14 at 12 55 17](https://f.cloud.github.com/assets/218391/2420796/f24f9318-ab77-11e3-878f-8d768b673e78.png)