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)

I think sending no-budgets here could create a little bit of confusion (there is budget available) as it looks to me that this is only there to trap the case when the bids are already in-flight while the slow mode gets activated. Even if this means busting the slow mode limit, it seems perfectly reasonable to me i.e. I would let those bids pass.