Move oguid to the package opengever.base.

Oguids are or will be used in multiple other packages, thus it should be available in the base package.

Furthermore do some cleanup, remove evil attribute name `id` and change parameter order to satisfy the common use case.

*no changelog entry necessary*
Stumbled across this commit while browsing the latest PR's... @deiferni I hope you don't mind a shy question :hamster::

Why would you combine two separate code path's into the same method and then separate them with conditionals and assertions? In my humble opinion it would make the code more straight forward if you had completely separate entry points. like:

```python
def __init__(self, id):
  pass

@classmethod
def build(cls, admin_unit_id, int_id):
  pass
```