Rework org unit selector, for better support of multi admin_unit setups

Reworked OrgUnit Selector  to avoid confusing situations in multi admin_unit setups:
- Store current unit to regular cookie instead of using the session_data_manager.
- Differentiate between users_units (selectable) and admin_unit's org_unit (current org_unit).
- Change current org_unit functionality, the current orgunit has to be part of the current admin_unit.

@deiferni please have a look ... 
you could subclass `DictMixin` (`from UserDict import DictMixin`) and then implement `__getitem__`, `__setitem__`, `__delitem__`, and `keys` for a complete dict implementation. The `has_key` method would not be necessary and could be written `KEY in storage` instead of `storage.has_key(KEY)`

a change like this would expose all cookies though ...