Finished distribution class

The distribution code that was recently merged into master wasn't the latest code. I had put the latest code on hold because I was waiting for phase 2 to be merged into master. But when it was merged, I forgot to update phase 3.
So here is the code, rebased on top of master.

It generates Distribution instances with a Peercoin address. The output is also made of Peercoin addresses.
I also added 2 helpers to simplify distribution: `GenerateDistribution` and `SendDistribution`.