(WIP) Graded library content - Do not merge, for internal review

This is a continuation of the prototype found in #2, extended to implement [SOL-19](https://openedx.atlassian.net/browse/SOL-19) "As a student, when I complete interactive exercises displayed in a LibraryContent block in the LMS, I see a single aggregated grade for the set of exercises in the progress tab."

Testing instructions: Follow same testing steps as #2, except on step 7 when adding content to the library, add problems (of any type). On step 10, be sure to set "Scored" to True. Then, after all the other steps, view the course as a student. Check that the "Progress" tab is correct, before and after submitting answers to the problems. Test the weighting and make sure none of the children appear in the progress view - only the combined score of the LibraryContent block.

**Library screenshot**:
![graded1](https://cloud.githubusercontent.com/assets/945577/4658555/d76d2f9c-54ff-11e4-8296-3682cb11663a.png)
**LMS screenshot:**
![graded2](https://cloud.githubusercontent.com/assets/945577/4658558/d99534a4-54ff-11e4-8dad-674143cae30e.png)

Randomize tells the system it has exactly one child (the one picked randomly). We always tell the grading system we have no children.

As you can see from the discussion on the wiki, it is up for discussion which behaviour is better.

Telling grading system we have no children:
* Pro: can re-weight the grades of child objects consistently - may sometimes improve consistency of assessments that consist of a LibraryContent block plus other assessment questions all put in the same unit.
* Con: "Progress" page combines all grades from children of the randomize module, potentially revealing less information to users.

Telling grading system we have [n] children:
* Pro: Progress page shows every single score.
* Con: Assessments that include a ContentLibrary block and other blocks may sometimes weight the ContentLibrary questions higher or lower for some students than for others.