feature/collections_using_redis_sorted_set - MultiSet on Redis SortedSet

Adds implementation of Counter on Redis SortedSets with better performance on 'most_common'. Also updated documentation. I also would like to know what others feel about inheriting from collections.MutableMapping abstract classes, as i think it might be a bit clearer in terms what should objects provide than simply writing methods. Lua implemention of in place operations will be added soon