Add a Borda Count polling type

Issue Summary

On an old issue, Flying Drupalist had a good idea to implement a Borda Count voting option.

I would love to be able to do ranking polls (http://en.wikipedia.org/wiki/Borda_count) with pollfield.

My idea for how it would work is, there is a poll setting "Count method" when the poll author is making the poll with radio buttons. The radios would be the various counting methods, whatever gets implemented.

This isn't a high priority right now, but I wanted to record it here as a feature request for future consideration.