Reworked module retrieval logic and data types

admin.py
	-Replaced json with simplejson from the django.utils library, will be removed later when module retrieval is fully segregated to a separate file
	-Added additional logging statements during debugging process

app.yaml
	-added the django library import, for its simplejson module

datatypes.py
	-Heavily modified existing classes
	-Most properties are now marked as required, and all attribute declarations have attribute names written for readability
	-Added the Skill Tree model, to allow a user to save multiple skill trees.
	-Added the ModulePrerequisiteGroup model, which can contain either additional nested groups or a list of module codes, as well as a type attribute to determine how the client will intepret the data from the sets of groups

mockup/css/main.css
	-added declaration for the NUSMods "link"

mockup/js/jquery.jsPlumb-1.4.1-all.js
	-added updated library for debugging

modulescraper.py
	-completely re-done logic for module retrieval; all external module retrieval and storage will be done by this file.