Filter GPS error points

It seems that the GPS data contains some errors. It's easy to see these if you change the time range to one week.

![screen shot 2014-01-21 at 1 39 05 pm](https://f.cloud.github.com/assets/429876/1963166/50fd8d62-8296-11e3-8b31-36d96fe42797.png)

In this PR I've implemented a very simple (and naive) filtering method to discard the points that are clearly wrong. The filtering method calculates the speed between two points and throws the points away if the speed is too much for snow plow (> 120 km/h).

This seems to fix the issue, but probably doesn't work if there are multiple error points that form a cluster. I guess in that case case some fancier GPS filtering/data analysis algorithm should be used instead.