Add property_callbacks to hook_field_info()

Looks good to me.

Rerolled so now it also supports rules and not just entity_metadata_wrapper().

In rules it is possible to reference field values in rules actions or conditions.

For example I have a project node type, which has a entity reference to a client node. That client node has a client code field which is using safeword. I could have an email list which is [client-code]@example.com and configure rules so every time something happens on the project node it sends an email to that email address.

I am struggling to come up with a better example that isn't directly derived from my client's direct use case.