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 ../
```