Add jasmine-rails to run JavaScript tests

For use in the JavaScript that is being written for the checkbox drop
downs.

- Add `jasimine-rails` gem to `Gemfile`
- Mount test runner on `/spec` so tests can be run in browser
- Add `jasmine_rails.rake` rake task so that tests are run by default
  when you call `bundle exec rake`
- Add a line to `test.rake` so the tests are run when you call
  `bundle exec rake test`
- Add a dummy test to prove concept, this should be removed when a real
  test is added
- Add copy of jQuery to helpers. This is normally included on pages
  using static/slimmer but won't be available in tests
- Add config for `jasmine-rails`
Merge pull request #7 from alphagov/add-jasmine-rails

Add jasmine-rails to run JavaScript tests