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*
@senny hm you're right this is ugly :confounded:.

i cannot extract `admin_unit_id` and `int_id` though, they are required by the sqlalchemy composite columns (http://docs.sqlalchemy.org/en/latest/orm/mapper_config.html#composite-column-types). But the oguid does not belong here, i would suggest something like this: https://github.com/4teamwork/opengever.core/pull/705