Implement course-wide asset paging methods.

Add SortedCollection class to help sort lists efficiently.
https://openedx.atlassian.net/browse/PLAT-74
It's not a separate package AFAIK - and I've already made modifications to the code I grabbed from the net. So I'll need to move it to common/lib. I couldn't find anything else existing in the codebase to use - do you know of anything?