Extending mainpage example

Yes, but I guess it depends what you are looking for. The background in the command line is an average background, the local level can be smaller, especially in a cluster, etc. Aiming to get all the sources I would set it lower. Attached is a plot using ``threshold = background + 3 * sigma``

![bkg_plus_3sig](https://cloud.githubusercontent.com/assets/6788290/3516476/9b2d2f30-06e1-11e4-8871-813b56fbb0ee.png)