Add report formats for download

Add methods for downloading CSV and NBE reports.
Add method for performing report chapter lookups.
Add methods for downloading HTML and PDF reports with chapter
selection (all by default).
Update plugin

Import nessus.rb plugin from framework PR @:
https://github.com/rapid7/metasploit-framework/pull/1621

Requires framework PR 1604 for full efficacy (Rex table search).
Merge pull request #9 from sempervictus/add_report_formats

Add report formats for download