HyperLogLog commands

I added hyperloglog commands and tests, please review. Unfortunately I had to disable tests, since Travis CI does not support redis>=2.89. If you to want to run test againsts new verstions of redis on Travis, consider following *.travis.yaml* config.
```yaml
before_script:
# patch redis config
- wget http://download.redis.io/redis-stable.tar.gz
- tar xvzf redis-stable.tar.gz
- cd redis-stable
- make
- export REDIS_PORT=6379 REDIS_SOCKET=/tmp/aioredis.sock
- >
  ./src/redis-server --daemonize yes
  --pidfile ./redis-server.pid
  --unixsocket $REDIS_SOCKET
  --port $REDIS_PORT
  --save ""
- sleep 3
- ./src/redis-cli -s $REDIS_SOCKET PING
- cd ../
```
Everything looks good.
But I would prefer `*values` split into two vars: `value, *values`
This would be more verbose when looking on function signature,
ie: `def pfadd(self, key, value, *values)` -- it's clear that at least on `value` is required and others are optional.
Same note regarding `pfcount` and `pfmerge`.