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 available [0].

[0] http://en.wikipedia.org/wiki/Hamming_weight

Change-Id: Ia05ca43a063ab74489213c00160e2dd217b7d0db
Reviewed-on: http://gerrit.causes.com/37000
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>
Upgrade RSpec from 2.6.0 -> 2.14.0

Change-Id: I4fed4098172c8cb9e9a67c5e82dc24db6d822df3
Reviewed-on: http://gerrit.causes.com/37001
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>