Package updates and general robustness improvements

I've made the following changes to update the package:

- Grab spin.js out of its NPM distribution, rather than copy the source to this repo. The most recent version is v2.0.1.
- Use the `.spinner-container` class for the template that renders the spinner, because `.spinner` is set on the spinner itself. Add CSS so that it takes up its entire parent, so the spinner is centered with default options.
- Make sure to `stop()` the spinner when the template is destroyed, as mentioned in #7.
- Make the code in the `rendered` callback more concise.
- Add an explicit dependency on `underscore`, because it's used in the `rendered` callback.