Added constraints and default for guid and rgt.
Added constraints and default for guid and rgt.

Guids now receives uuid.uuid4() per default value.
RGT now receives LFT+1 as default value.

Constraints added for supporte Nested Sets Hierarchy added. The job is not complete yet.

gcavalcante8808/flask-wiki,gcavalcante8808/flask-wiki,gcavalcante8808/flask-wiki