Rebuild of resource_conflict using Rules and Entities

I'd like to rebuild this module around Rules and Entities, to make it easier for people to customize the workflow and results. Before I start committing, I'd like to open the floor for discussion on this. The structure I'm thinking about is simple:

* Make "conflict enabled" a generic entity property. So any entity can have conflict detection enabled.
* Build a single Rules event for "conflict detected", which matches any time two entities with conflict detection have conflicting dates (regardless of 'resources' reserved)
* Build the same (current) workflow in Rules and bundle them with the module.

So this module would be greatly simplified, down to the ability to enable conflict detection, select the relevant date field per-conflict-entity, and throw a Rules Event when they conflict. Rules would be included so that by default, a form error is thrown whenever a date conflict is detected. With Rules, it's very easy to add resource type detection into that workflow, and have different responses based on a variety of conditions.

Use cases not possible with the current structure:

* multiple conflict types: ie 2 kinds of events which should not conflict with each other
* conflicts between any field values, ie display a form error when a conflict pair shares the same TID.
* complex resource conflicts: ie a form error should be set when a conflict pair shares same taxonomy term AND the same entityreference value.
* different reactions per conflict: ie when a conflict pair shares the same TID, throw a form error. When it shares an entityreference, send an email and log it.

Looking through the issue queue, a lot of problems can be solved with this different structure.

On the down side, it would be more or less a complete rebuild. The core date comparison code would stay, but that's about it.