To be 'mongoid-slug', notice the hyphen. Issue #179

Naming fix for the new gem, with hyphen (to be registered with rubygems).
My suggestion is that the current be moved to a underscore branch, keeps the github name correct.
And a new release of the old gem with deprecated notice. #179
It is decided already there.

Anyway I just copy-pasted text for inspiration. Feel free to use better text.