Add method scan_by_checksum for virustotal.rb

Allows the user to scan files based on checksusm (without actually
uploading them to VT)
Put classes under a module
Fix VirusTotalUtility module scope
Strictly just -q for the quick option
Change options. And change "checksum" to "hash"
Land #2867, @wchen-r7's changes to the virustotal tool