updated setup.py
Updated API and Implimented Weighted Sampling

1. changed the `.feed` method to an `.append` (I think its more clear
what it should do. Its just a probabistic data structure now.

2. It is important to note that there are two interpretations for a
weighted probability. I have implemented the sampler that views weights
to be absolute where (2, 'firstelement') has twice the probability of
being in the reservoir as (1, 'secondelemebt').