Slow mode money limit

Change the slow mode from a QPS hard coded limit to a authorized money limit that is settable as an option in the launch script.

I tested it through forcing slow mode and looking at the cummulatedBidPrice. 
Here the first test was with 100 CPM as limit, though that is too low to bid, then with 1000, next 2000, 5000 and finally without a limit.
You can see it does what is expected.

![testing_slow_mode](https://cloud.githubusercontent.com/assets/2280752/4208018/59d4d360-385c-11e4-9255-c5cf93635f88.png)

since we had a default (well, more like hard limit) of 100 qps, it's probably a good idea to provide a default value here. if we assume $1 CPM, we could limit to $100 CPM i.e. 0.1 $/s.