Add HMGET support and WITHSCORES support for ZRANGE

Added HMGET support, and added WITHSCORES support for ZRANGE, with tests and readme update.

I also refactored the ZRANGEBYSCORE code a bit so that ZRANGEBYSCORE and ZREVRANGEBYSCORE could share code (sorry for the messy diff).
Could you also test `hmget` with fields and keys that doesn't exist ?