Implement Associations

Common associations are now implemented between domain objects. This
allows multiple objects to be associated to one another using native
python lists as properties.
Implement Many-To-Many Sprint Task Association

Each sprint can have many tasks and tasks may span many sprints.
Add Missing Properties

Story points and hours are now attributed to UserStories and Tasks
repectively.
Add New Properties to Schema in SQLPersistance

Store new properties in database.
Implement Hour Functions

User Stories can now aggregate hour counts for associated tasks.
Implement Test Example for Models

A simple test to make sure the aggregation functions of UserStories
function correctly.