Add ability to ignore unlisted users

![if you don't like that, send it right back](http://us105fm.com/files/2013/12/anchorm.gif)

This allows gratan to be run on databases that have users managed in other ways by specifying an option that it should only attempt to drop users that are listed in some way in the Grantfile.  This also adds an option in the dsl to specify users as dropped.

My use case is that I have a bunch of databases with users managed in various ways.  I'd like to start managing some accounts with gratan (or something), but I'm not ready, willing, or able to think about managing all accounts in this way.

:guitar::+1::dancer: