Add widgets shortcuts

##### Updated

Summary: The user can type directly in the textarea the widgets' shortcuts and they will be replaced on the fly by the widget placeholder. Also, widget insertion in mid-text!

![new-shortcuts](https://cloud.githubusercontent.com/assets/6400898/4177059/a9a5b584-3623-11e4-910b-ac07e4e9368c.gif)


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Summary: Quick insert of widgets into the editor via keyboard shortcuts

![shortcuts3](https://cloud.githubusercontent.com/assets/6400898/4138904/9436cc6e-3395-11e4-8555-117f5c33b7fd.gif)

How it works:
1. While writing the question/answer/hint, tap CTRL
2. You have now half a second to type the shortcut of a widget to quickly add it (for example: 'N' for the number line)
3. Press spacebar to immediately insert the widget, or just wait
4. Rinse & repeat

Thought this could be useful for someone who writes a lot of exercises :)

What has been done
* Added a onkeyUp handler to detect the CTRL key and to start the capturing of the shortcut
* Added a onkeyDown handler to capture the shortcut characters
* Added a method to Util for simulating events
* Extracted a shortcut of each widget based on its name, and displayed it on the widgets dropdown
* Added a info tip to notify the user of the shortcut feature
* Fixed a discrepancy between FF and Chrome with the margins of the <select>s in .perseus-editor-widgets-selectors
Not sure what you mean by "instead of triggering an event elsewhere"? My aim was to reuse the event handlers already in place instead of writing additional logic, that's why i went with simulating the change event on the widget dropdown. How would you like it to be done instead?