Provisional support for OpenDNS Investigate API

[OpenDNS Investigate](http://investigate.opendns.com)* API requires an Authentication header (HTTP Basic Auth.)  Seemed easiest to just allow Headers to be specified for GET or POST requests. `INVESTIGATE_API_TOKEN` needs to be changed to a token generated on the OpenDNS Investigate site.

Full disclosure: @ehwhydubya and I are employed by OpenDNS and worked on this for an OpenDNS Hack-A-Thon project.

\* Access requires a paid subscription to the service.