Enable serialization of data entity and collection.

This PR enables the serialization of data entity and collection. 

Serialization was prevented by unserializable properties that held
`PDOStatement` or anonymous functions.

Both entity and collection objects cannot be serialized losslessly. As
it however is assumed that serialized/unserialized objects are used
just before they are injected into views and no further complex
action will take place these losses will not be noticed.

* Both objects will loose any associcated method filters.
  As none of nore subclasses of them actually use filters for now
  this is a non-issue.

* Entity will loose schema. 

* Just the default export handlers are restored.

Data experts (i.e. @jails), do you see problems with entity loosing schema in the scenario described above (caching for the view)?