Nessus support!

This PR resolves #2 adding support to parse Nessus reports and download reports from a single Nessus server as defined in the user profile. A separate issue will be raised to add support for multiple Nessus servers.

There are also a few minor bug fixes and small mods required to other parts of the code to get this working. Most notably we use the concept of 0-10 for severity levels whereas Nessus uses 4 (Info, Low, Med, High, Critical). In order to support this I have them mapped them to the following:

    sevmap = {'0': 0, '1': 3 , '2': 5, '3': 8, '4': 10}

The graphs look a bit weird because of this - I'll open an issue to allow for CVSS Score in the graphs instead of Severity.

Overall this should satisfy your basic Nessus and OpenVAS (untested) usage.