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
How about:
`"[NOTICE]: Deprecated gem name. The name of this gem will be changing from  'mongoid_slug' to 'mongoid-slug' (notice the hyphen). To ensure you are using the correct version of this gem, please use 'mongoid-slug'."`

Thoughts?