Make userpoints_transaction entity fieldable

Working proof of concept.

The patch adds a default bundle, exposes manage fields/display forms through that, implements the necessary field_attach_*() functions and also contains some improvements regarding the new API, for example an __isset() magic method so that isset() and empty() work as expected.

It is possible to add new fields, create transactions with fields and display them. However, they can't be properly arranged yet, we first need to expose all existing properties as extra fields. And since most of them reside within vertical tabs, I want to research if it is possible to expose them to field_group, so that they can be moved around in any possible way.

Also, I certainly want to add some tests for this, probably port those from Privatemsg over.

New patch, fixes the test fails.

Also implements hook_field_extra_fields() for the edit form. Right now, the whole vertical tabs is a single pseudo-field that you can move around but that's about it. We're currently discussing on how to extend field_group to allow us to make the different vertical_tabs visible in the UI. See #1280974: Allow to show custom in code declared elements as field_group in the UI.

Erm, and now with patch!

#5: 1258018_2.patch queued for re-testing.

#5: 1258018_2.patch queued for re-testing.

#5: 1258018_2.patch queued for re-testing.

New patch

- Rerolled to incorporate the entity.module changes
- Removed the field group stuff, that probably has to wait. I'm in the mood of pushing this a bit along and we don't need super-fancy vertical tabs integration yet ;)
- Add simple tests to make sure that fields are displayed.

Commited.