Fields corrections

Close #67 "Redis.py does not support descrby" (incrby too)
Close #68 "setex and indexation"
Close #69 "Change behaviour of InstanceHashField.hmget"
Close #70 "setbit is not tested" (incrbyfloat was not really tested)
Close #71 "Add missing redis-hash commands to models for InstanceHashField"

+ add missing tests and commands