use custom binding for knockout.js enter key

[benatkin]

I think it would be more idiomatic Knockout.js code if you set up a custom binding rather than check the keycode in the add function.

The custom binding page in the docs has an example of an event binding under hasFocus(). It's jQuery. It might be better for example purposes to use Knockout functions to set up the event.

[boushley]

Fixed in pull request #48.