Implement course-wide asset paging methods.

Add SortedCollection class to help sort lists efficiently.
https://openedx.atlassian.net/browse/PLAT-74
I guess these tests are me doing the old "trust but verify". If I assume that this module does what it purports to do, then you're right, there's no point in having these tests bloating the codebase. I'll get rid of them.