Add ns_goodmail for whitelist NickServ emails

This adds support for whitelisting email domains allowed to register
nicks with NickServ.  It is the opposite of the included badmail module.
Add help text for ns_goodmail module

I wasn't sure how to contribute help files to the contrib repo.
Feel free to delete this if it doesn't make sense.
Use GE verb for ns_goodmail database storage
Merge pull request #19 from blt04/ns_goodmail

Add ns_goodmail module