JavaScript testing

This update adds [Konacha] for testing JavaScript behaviour. A consumer should explicitly require the dependency in their Gemfile:

```ruby
group :test, :development do
  gem 'mas-development_dependencies', require: 'mas/development_dependencies/konacha'
end
```

And then run the tests:

```sh
$ cd spec/dummy
$ bundle
$ bundle exec rake spec:javascript
```

The demo app has been updated to reflect this and is now isolated with it's own Gemfile. The demo app contains both JavaScript and CoffeeScript examples.

[konacha]: https://github.com/jfirebaugh/konacha