Add replications option to bridgedb.hashring.Hashring class.

Setting `replications` to >1 will cause an item to be inserted that
number of times into the hashring, distributed uniformly. I haven't
decided if this should be part of the `b.hashring.Hashring` class, or
part of another, separate class (since technically what this implements
is "consistent hashing"). [0]

 * ADD `replications` argument to `bridgedb.hashring.Hashring`.

[0]: http://www.tomkleinpeter.com/2008/03/17/programmers-toolbox-part-3-consistent-hashing/