Add basic whitelist support

This adds basic support for whitelists as described in the ClamAV
developer manual, "Creating signatures for ClamAV."  It creates three
file resources, `/var/lib/clamav/local.ign2`, `/var/lib/clamav/local.sfp`
and `/var/lib/clamav/local.fp.  Each takes an array via a parameter to
the init class.
Use absolute path for clamscan binary, accept parameter to set it