WIP consolidate markdown lines into chunks. Blank lines not working yet.
Fix markdown rendering of new lines
WIP: Introduce the concept of editing and editingIndex,

- set up keyboard shortcuts to toggle appropriately
- stub a callback to save new content
WIP: move save and esc shortcuts to WorksheetItem, add keys to array items, style textareas for editing
Move esc and save methods into MarkdownBundle,

move consolidateMarkdownBundles into WorksheetContent so we have one object of truth to pass around
Code cleanup
#491

- refactor Worksheet class and markup bundle consolidation to store state in global ws_obj object
- add WorksheetItem class
- markup bundles are editable and ready to be saved
- update MathJax cdn address per http://www.mathjax.org/changes-to-the-mathjax-cdn/
Merge branch 'refs/heads/feature/491-edit-markdown-in-worksheet' into upstream/feature/worksheet_interface

Conflicts:
	codalab/apps/web/templates/web/worksheets/detail.html