Show unique number of networks in metrics

This PR is a bit larger, I am looking forward to get your feedback.
I read a German article about the Mozilla Stumbler release, and the author wrote that he had stumbled over 900 wifis in a really small area, so I remembered #1019.

It's best to check each commit separately:
- first commit does some cleanup and renames some variables, no functional change.
- second commit makes the `Reporter` count the observations, instead of the `MainApp`
- third commit counts the unique networks in the `Reporter`, I guess it fixes #132
- fourth commits displays the unique network stats in the metrics (see first screenshot)
- last commit removes the other network counts (see final screenshot)

This was one of my tests to see that the code works. I did five scans on the same position (6 cells, 7 wifis visible), only two of the scans had wifi enabled:
![screenshot_2014-11-07-00-15-45](https://cloud.githubusercontent.com/assets/3845150/4946052/1b78a6d8-660f-11e4-8dd3-e8d9bdbab090.png)

This is the final version:
![screenshot_2014-11-07-00-25-24](https://cloud.githubusercontent.com/assets/3845150/4946069/4f5858ae-660f-11e4-8d27-7e380c78b8fb.png)