Add mongoid support

This came up a few years ago (see #16 and #18) but I thought I'd try again as I see workflow now has separate adapter classes, which makes it a bit easier to add new adapters.

It's actually a bit of a pain to keep [workflow_on_mongoid](https://github.com/bowsersenior/workflow_on_mongoid) up to date with changes in workflow (see https://github.com/bowsersenior/workflow_on_mongoid/pull/12), so I thought it would be logical to add the small `Mongoid` adapter to workflow itself.

Let me know your thoughts. Thanks!