Uhashring

* split ring implementations into modules, faster md5 ring rewrite
* switch to md5 implementation as default, change the hashring signature
* Merge pull request 7 from ultrabug/mod_rings_and_new_sig
* add ketama collision test thx to bjhockley wrt issue 6

* fix python3 python-memcached monkey patching and tests thx to garrylachman, fixes 5

* update the README for user defined hash function
* dont force vnodes and replicas when using an external hash function

* rename _hashi_faster to _hashi_md5
* add new hash_fn option to allow passing a custom hash function for the ring

* Avoid the exception AttributeError: can't set attribute, in the case of executed test suite with an version of setuptools greater than 18.3, by Babacar Tall
* unit test: Add the compatibility for python 3, by Babacar Tall
* Adding of envtest=py35 in the tox.ini, by Babacar Tall
* Added tox.ini file in the MANIFEST.in, by Babacar Tall
* Merge pull request 2 from btall/fix/python3-setup-test
* fix ridiculous time error
* add hashes simple benchmarking code
* add tests on ring growth
* performance: avoid ring recalculation

* 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