Merge pull request #1934 from minrk/cellmd Cell/Worksheet metadata * metadata dicts are attached to cells and worksheets * restores collapsed flag to the nbformat - this change happened in the refactor, and was undocumented, and possibly accidental. But we should either document it or fix it, and this includes a fix. * adds a new field, `nbformat_minor`, used to denote minor bumps of the notebook format that expose new capabilities but don't prevent loading by older clients. * Add a warning in Javascript if loading a multiworksheet notebook (which will exist in the future) as current JS code will only save the first. closes #1915