Restructure parts of Author and fix some doc bugs.

This adds specs to `Msf::Author`, reworks some logic and documentation in `Msf::Author`, renames the `Known` constant to `KNOWN`, and fixes a few yardoc issues in other classes.

#### Verification
- [ ] Travis passes
- [ ] run `info` on a module from `msfconsole`
- [ ] authors show up as expected