Fauxton: Fix status messages in Active Tasks

Show Progress, current Sequence and Changes. Fix the display
of `undefined` if value is undefined.

Fixes #COUCHDB-2123

Before (click to zoom):
![bildschirmfoto 2014-03-05 um 21 31 25](https://f.cloud.github.com/assets/298166/2338161/264fb39c-a4a6-11e3-8d98-b281d9bda16d.png)

After (click to zoom):
![bildschirmfoto 2014-03-05 um 21 31 01](https://f.cloud.github.com/assets/298166/2338165/36e98d68-a4a6-11e3-8a88-defe2009cbd7.png)