Enable downloading of external rulesets (NBS, Magemojo, Byte)

[gwillem]

# mwscan -s nbs /data/all-magento/magento-2.0.6 --deep
Tue Jan 17 15:11:33 2017 Using NBS rules.
Tue Jan 17 15:11:33 2017 Fetching php.yar
Tue Jan 17 15:11:33 2017 Fetching whitelist.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/drupal.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/wordpress.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/symfony.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/phpmyadmin.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/magento2.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/prestashop.yar
Tue Jan 17 15:11:34 2017 Fetching whitelists/custom.yar
Tue Jan 17 15:11:34 2017 Fetching common.yar
Tue Jan 17 15:11:34 2017 Loaded 15 yara rules and 1279 whitelist entries
Tue Jan 17 15:11:48 2017 Finished scanning 41514 files: 76 malware and 25 whitelisted.

Todo:

• Fix relative import failures for Python 3 in tests