Hacky convert of calculator sample app to use Tavern.

.. because I wanted something to play with in Tavern :p

This deletes calculator's copy of jquery and adds a tavspec.yaml that
pulls it in from Tavern. It "works", but:
 - Calculator isn't tested with jquery 1.10 (only 1.6) so it throws some
weird cruft into the UI
 - Had to add to the .gitignore: cache and pubspec.lock that shouldn't be
there (eventually)
 - Had to hardcode jquery version spec in the html - obvs will break if
the package version updates

Otherwise: \o/

R=pajamallama@google.com

Committed: https://github.com/tapted/chrome-app-samples/commit/d5db60c

Review URL: https://codereview.appspot.com/51080043