Add configuration for running tests with Jest

Can be run with `node_modules/.bin/jest` for now; didn't want to disturb the grunt setup.

Right now one test fails with:

```
 FAIL  browser/ui/__tests__/ReactDOMComponent-test.js (1.423s)
● ReactDOMComponent › updateDOM › it should update styles when mutating style object
  - Expected: '0' toEqual: '0.5'
        at Spec.<anonymous> (src/browser/ui/__tests__/ReactDOMComponent-test.js:99:33)
```

which I can only assume is a jsdom problem -- no other asserts fail.