Environment Variables

It's a core tenant of the [12 Factor app that configuration should not be stored in code](http://12factor.net). Given this is especially key with authentication tokens such as API tokens I pulled the Malware.lu & VirusTotal API keys out of the code and suggested they be added as environment variables. 

If they're available they'll be read in, otherwise they'll be assigned the primitive ```False``` causing the same failure condition as before. I also updated the documentation to make sure users understand how to set this up.