Added Constraint{'', type, set} migrations
New gxml parser library is now parsing courses nodes
Added new library files
gxml parser library is now parsing edges nodes
gxml parser library is now parsing edges nodes
Entities are now correctly created when parsed
Parser is now creating binary constraint with the new constraint model
Refactored create_binary_constraint method
Moved create_constraint_type & set_type to the coresponding model
Moved create_constraint method to the coresp. model
Finished Parser refactoring
Added catalog_id to constraint_type & constraint_set_type model