Add support for BITCOUNT command

`bitcount key [start, end]` counts the number of set bits (population count) of a string and has been available since 2.6.0.

The 'naive' implementation of the Hamming weight algorithm is used, although more sophisticated implementations are availble [0].

[0] [http://en.wikipedia.org/wiki/Hamming_weight](http://en.wikipedia.org/wiki/Hamming_weight)