More fixes on the Qiita(Status)Object classes

Some fixes on the QiitaObject and QiitaStatusObject base classes:
- The `__init__` on the QiitaObject class now checks that the `id` passed actually belongs to an object in the database.
- The status getter/setter can be written in the base class. Just need to define a new function `_status_setter_checks` that should be overwritten by the subclasses in order to perform the necessary checks before updating the DB object.
- The `check_status` decorator was not working. Rewriting it to be just a boolean function, so subclasses can call it when needed.
- The documentation in the entire file was not following the numpy standard. I've updated the documentation.

It would be great if this can be in soon, as it should be merged before any other object is merged.