Add code folding

This PR Fixes #621.  This is preliminary right now, just to get some comments going.  Pulls all the code for the different folding addons from CodeMirror, located here: http://codemirror.net/addon/fold/

Currently just added simple editor user behaviors "fold-gutter" and "hide-fold-gutter" (with default set to hide) to show folds on gutters.  The auto method is used and default CodeMirror helpers included in fold.js to setup folds.

Next need to set up keybinding to create user folds.