Updates to the home page

This PR adds functionality to the home page example for the website. Specifically, it adds the following:

* Inline editing for the number of books. As you change the number, the text changes. This is important to demo pluralization.
* An **About** link, which when clicked, shows a panel with some content on the ICU Formatted String that makes the example work.
* A **Build this example** link, which when clicked, takes you to `/start/handlebars` where the user can construct a very similar example (without inline editing). The example that the user constructs can be seen at `/start/handlebars/rendered`

## Screenshots

### Inline-editing
![](http://f.cl.ly/items/2H2V2A3Z2F0U0Y0w2A1a/Screen%20Shot%202014-07-31%20at%205.51.35%20PM.png)

### About Panel
![](http://f.cl.ly/items/1L2I153B393Y351s2C0B/Screen%20Shot%202014-07-31%20at%205.51.55%20PM.png)

## TODOs
I think the following needs to be done before this PR can be merged.

* [ ] The PR adds HTML to the ICU Formatted String (called `USER_HAS_BOOKS_DENOTED` and `USER_WILL_SELL_DENOTED`. These strings only exist on `en-US`, `es-AR` and `fr-FR`, but need to be added to the other languages too, so that the dropdown works properly.
* [ ] I recommend setting the default number of books to `1`, so that it's easier to see pluralization in the works.
* [ ] Better number validation for `<input type="text">`. Btw, the reason I didn't use `input[type=number]` is because it was messing up with numeric `value` having commas (putting `value="2,000"` wasn't working well).`