Show estimated user position accuracy error.

Accomplished by averaging the differences between the actual distances and RSSI-estimated distances between the user position and the beacons.

![screenshot_2014-04-22-23-38-12](https://cloud.githubusercontent.com/assets/2092627/2773434/61a65684-ca9a-11e3-8990-ad0640ddffd2.png)
![screenshot_2014-04-22-23-37-45](https://cloud.githubusercontent.com/assets/2092627/2773436/61a7746a-ca9a-11e3-9399-860f3da3635c.png)
![screenshot_2014-04-22-23-37-18](https://cloud.githubusercontent.com/assets/2092627/2773435/61a740b2-ca9a-11e3-968a-85fc6abae5df.png)