Load balancing based on score.

Added the ability to load balance based on `least-score` or `most-score`. This allows redx to load balance the backends based on a score you use (ie number of connections, load avg, memory free, number of threads opens, etc). See `README` changes for more information. 

In order for this to work, you do need to update the score via the API on a per backend basis routinely (ie once a min).

Picking a backend randomly is of course still available and will continue to work with no changes (backwards compatible).