Implement Mobile Editor

This is working I think but I don't know how to get the tests to pass.

closes #2957
- add FastClick library to Gruntfile.js
- add touch-editor to client/assets/lib/
- add mobile-specific utils to util/mobile-utils.js
- add codemirror util to set up TouchEditor only if we're really on mobile
- change gh-codemirror from having a default action to a named action. prevents Ember.TextArea firing action on change
- change gh-codemirror `cm.getDoc().getValue()` to `cm.getValue()` for portability
- change codemirror-shortcuts ES6 export/import style
- changed ghostimagepreview.js to check for window.touchEditor in addition to Ghost.touchEditor
haters gotta hate ;)