Implement more of the api

[jeremybuis]

We currently have a very small amount of the api implemented.

It would be useful to have more of it done. Not sure on what api calls to add specifically.

More of nessus.scans, nessus.editor, nessus.folders, nessus.policies, nessus.scanners, and nessus.session would be good, not in that order. scans, editor and session are probably highest priority.

@dg3kim get nessus installed on your machine and have a look at the docs and how this library works. If it helps add an examples folder with sample api calls, otherwise look at the tests to see how it works.