Separate oguid entry points.

Add a parse method to create oguids from their string-representation. Based on feedback from @senny in https://github.com/4teamwork/opengever.core/pull/701#event-204278137
The database query methods can take either a string or another Oguid at the moment,  e.g.: https://github.com/4teamwork/opengever.core/blob/deif-oguid-rewrite/opengever/globalindex/model/query.py#L43