Add revision message during state change

Thanks for the patch! This solved a big issue for me.

Here's a slightly revised patch with the following changes:

1. Removes the alert() of result.message on successful change - this is simply just annoying to the user if already familiar with the likes of Trello etc
2. Simplified the revision log message stored - there is no need for the entity type or the entity id as it is saved against the entity changed.

Removes the alert() of result.message on successful change - this is simply just annoying to the user if already familiar with the likes of Trello etc

This is unrelated to the current issue, so please create a new issue for that.

Another thing: creating new revisions for state changes should probably be optional, so I think a new configuration option is in order here.