Crowdsourced Hints - "0.2 release"

Applies a bunch of improvements to the crowdsourced hints xmodule.
- Adds support for formula response types.  To do this, the formula response has been refactored slightly, to expose some internal functionality.
- Works with rounding tolerances specified in each response type.
- Hint-collecting UI now displays all hints that the user saw on one view.
- Closes numerous 500 errors.
@rocha