Karma test runner

Run the batman.js test suite using [Karma](http://karma-runner.github.io/).

This works on Travis CI.

It runs the test suite with a REAL DOM (not the fake imitation that is jsdom).

It runs the test suite on multiple browsers at once.

It runs the test suite in 19 seconds.

It runs the test suite on every file save (if you wish).

We get to remove all our `if IN_NODE /* skip these tests */` conditionals.

It's what Shopify is moving to.

Yay!

Thoughts? @nickjs @pushrax @hornairs @matthelm @cjoudrey @kristianpd