Add support for HINCRBYFLOAT

This commit adds HINCRBYFLOAT (available since Redis 2.6.0).
The code and specs were heavily inspired by the ones already
available for `hincrby` and `incrbyfloat`.

Change-Id: Id4c1c9be22b5f315fdbe945ab23166ca7b3e56db
Reviewed-on: https://gerrit.causes.com/33200
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>