Fix storage of preference scoped XBlock fields (issue #7).

This explicitly handles the preferences and user_info scopes
in the model key lookup. It also adds AllScopesBlock, which
will exercise all scopes except children. Since we cycle
through all scenarios to make sure they don't cause load errors
in our tests, this should catch the problem in the future.
Merge pull request #12 from edx/ormsbee/fix_prefs_storage

Fix storage of preference scoped XBlock fields