Implement Pagination for Content libraries (SOL-4)

* **Background**: This PR provides pagination for content Libraries in the container view, when displaying the blocks contained in the library.
* **Jira tickets**: Implements SOL-4
* **Discussions**: Architecture discussed extensively on the wiki and in meetings, then the revised proposal was presented to the Arch Council on Oct. 21 and given thumbs up.
* **Dependencies**: https://github.com/edx/edx-platform/pull/6046 (merged, in feature branch https://github.com/edx/edx-platform/commits/content-libraries)
* **Sandbox URL**: http://sandbox.opencraft.com:18010/library/library-v1:TST-X-01+now
* **Internal code reviews PRs**: https://github.com/open-craft/edx-platform/pull/9 https://github.com/open-craft/edx-platform/pull/18
* **Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

@cahrens I think the reason this is being done is because there's a lot of unclear overlap in responsibility between the ContainerView and Pagination. Backbone's paginator is expected to handle paging, contacting the server for assets and pulling them down in some consistent manner, but there are a number of events which might confuse it due to the way the container view refreshes and populates itself. The problem seems to be that getting the real backbone paginator to work would either involve very heavily customizing the paginator or rewriting a lot of container code, and that having a 'cheater' interface that will work with PagingHeader and PagingFooter avoids a lot of that.

@e-kolpakov Is that what you were trying to do? Am I understanding this right? I've only been using Backbone for a week now, and only knew that's what it was two days ago. :)