Restructure parts of Msf::Author, add specs, and fix some doc bugs.

This adds more 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
- [x] Travis passes
- [x] run `info` on a module from `msfconsole`
- [x] authors show up as expected