Added concurrent.futures to risklib (temporary solution)

The plan (according to Matteo) is to put the debian package python-concurrent.futures in our PPA. At the moment our PPA is not ready, so temporarily I am putting the code in our repository. We will remove it as soon as the PPA will be ready. Notice that the package  python-concurrent.futures is in the official packages of Ubuntu 12.04, but in precise-backports and Matteo says that it is bad to require our users to enable the backports.
That's the reason for this workaround.