added the create_notice_type wrapper in models

In the docs it is advised to use create_notice_type to create a new NoticeType object. But this function was absent. Although it is possible to create a new NoticeType using the 'create' function of the class, this addition makes the code follow the docs and compatible with previous code and other apps.
Merge pull request #16 from Bastien-/master

added a create_notice_type wrapper in the models