Bret Victor style input widgets

This PR implements what I am calling "input widgets":

* The allow a user to edit code in a code cell using a UI control (slider, etc.).
* The on change event of the UI control is bound to cell execution.

Input widgets are triggered by a user selecting text in a code cell and then pressing `Alt`. The selected text is then tested against a registry of regular expression to call the right widget. There is a small amount of code in `CodeCell` that I have added to register the input widgets and dispatch to the right one. Everything else is in a new `inputwidget.js` file.  As an example I have created a slider to adjust the values of integers.  There are a couple of things that need discussion

- [ ] Do we like the overall design of the `CodeCell` part of this?
- [ ] Do we want to ship any input widgets by default with IPython?
- [ ] Do we like the user experience of the current slider example?

Here is a simple screenshot of what this looks like:

![screen shot 2013-10-04 at 7 08 13 pm](https://f.cloud.github.com/assets/27600/1273991/06c6cc6e-2d63-11e3-8ddf-036dee380dbc.png)