Lazily load API objects

[jessemcbride]

In some cases, two API calls are made by CanvasAPI when a single call would suffice.

For example, to get all of the users in a course, CanvasAPI requires the following:

course = canvas.get_course(123)
users = course.get_users()

It would be nice to condense this (and others like it) into one call where possible.

We'll likely need to rework CanvasObject to support this behavior.