Add HyperLogLog commands (#431)

Hi All,

I have added HyperLogLog commands. Can anyone check PR (especially distributed part)?

Thanks
Whoops. It looks like redis 2.8.9 accepts multiple keys for pfcount, so this should be:

~~~ ruby
# @param [String] *keys
# @return [Fixnum]
def pfcount(*keys)
  synchronize do |client|
    client.call([:pfcount, *keys])
  end
end
~~~