Uhashring

* allow benchmark to run without ketama
* map has better performance than comprehension list thx to btall
* implement a weight_fn parameter allowing users to pass their own weight calculation function to the hash ring
* add tests for weight_fn implementation
* document and demonstrate the weight_fn implementation

* add get_instances method to get a list of all available instances on the ring
* ring.nodes is an alias of ring.conf for convenience