DNSWL support (via RBL) added

Hi! I added support for DNS whitelisting (via RBL). But this would require changes in rbl.lua for returncodes comparation. Perhaps this can be done better.

http://dnswl.org/tech

Return codes
The return codes are structured as 127.0.x.y, with "x" indicating the category of an entry and "y" indicating how trustworthy an entry has been judged.

Trustworthiness / Score (127.0.x.Y):

    0 = none - only avoid outright blocking (eg Hotmail, Yahoo mailservers, -0.1)
    1 = low - reduce chance of false positives (-1.0)
    2 = medium - make sure to avoid false positives but allow override for clear cases (-10.0)
    3 = high - avoid override (-100.0).