Fix generate.py to work with new client.csv

Hi there,

I reworked the `getClientCountryChoices` Methods in generate.py to work with the new client.csv statistics file. As mentioned in this [issue](https://github.com/shadow/shadow/issues/213) the direct-users.csv is deprecaed and is no longer available. If this request gets merged, the scallion-plugin wiki entry has to change slightly at the passage where the creation of an own simulation is described.

The clients.csv is allways online at the following url: https://metrics.torproject.org/stats/clients.csv