#20 Include pigz+gzip combined driver, set it as default

[Benedikt Hallinger]

Hi Gundolf,
i wrote a custom driver that uses pigz in case it is installed on the system.
If it is not installed, gzip will be used instead. This gives a huge speed boost in case of multiple cores and should be fully compatible with the old behavior.
To enable pigz, a user just has to have pigz installed on his system and does not need to alter the backup configuration, allowing him to extract archives on systems without pigz in place (like after full restore on clean systems). If gzip for backup purposes is needed explicitely, the old GZIP-only driver can be used.
On single core machines pigz is usually not present (as it doesnt make sense here).

Hereby i want to propose to you to include the attached patch, adding the switching driver to the built-in types and also setting it as default driver.
In case you want not to alter the default driver, it would be good to include the driver but not make it default.
Please also add a comment to the example config file to announce the driver and its speedier behavior under multiple cores.