Fix #39

- At initialization, fall back to RoundRobinRule when there is not enough statistics.
- Changed the weight to be (sum of response time of all servers - response time) to give more weight to servers with shorter response time.
- Changed to use the average response time as opposed to 95 percentile due to a netflix-commons bug (https://github.com/Netflix/netflix-commons/issues/3)
- Deprecate ResponseTimeWeightedRule as it is duplicate of WeightedResponseTimeRule