Add support for PTTL command

Implements the `pttl` command as defined in:

http://redis.io/commands/pttl

Change-Id: Icf2fbeda0e50b17c7942f35ea365f89d5fd168e4
Reviewed-on: https://gerrit.causes.com/33212
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>