upgrade to codemirror 2.35
Fix help page
Fix tutorial
Merge pull request #13 from tkluck/codemirror-2.35

upgrade to codemirror 2.35
upgrade CodeMirror to 3.0
Fix undefined test bug
Keyboard navigation between cells
Make sure that TinyMCE configuration is the same as the official nb
Remove periodic refresh of worksheet list
Removed notebook_lib.js
Minor typo corrections in worksheet.js
Add account creation error message
Merge master from @samuela