Add support for `npm test`

I already have npm installed [globally] on my system...it's annoying that I have to install something else just for react in order to run react tests.

Of course I could just do `node_modules/.bin/grunt test` -- but that's long and there's an alternative to all of this: `npm test`

This effectively aliases `npm test` to `grunt test` to make react's repo at least pretend to be more like most other repos on npm.