Initial commit of HyperLogLog commands

This is the initial commit of the HyperLogLog probabilistic counting
command introduced in Redis.

Support for the following commands is implemented

  * PFADD <key> <member1> <member2> ... <memberN>
  * PFCOUNT <key>
  * PFMERGE <dstkey> <srckey1> <srckey2> ... <srckeyN>
Add an additional test function to test.php
Use efree on a zval, not STR_FREE() :)
Calculate the proper minimum argc.  :)
Merge branch 'feature/hyperloglog' into develop