Merge pull request #1303 from ellisonbg/updatecm This updates CodeMirror and refactors a good bit of the notebook code related to it. * Updated CodeMirror to the latest stable release. * Fix numerous bugs related to the CM update. * Refactored the Cell API and the notebook's cell handling methods. * Generalized split/merge to work with all cell types. * Generalized "Edit in Ace" to work with all cell types. * Loading optimizations: pager starts out hidden, faster loads. * Shading added to Markdown and HTML cells when they are being edited. * This branch will require solid usability testing on Safari, FF and Chrome before merging. * Fixed a number of CM related bugs.