Prepare for publishing to NPM

* Update `package.json`:
  * Add `main` field, pointing to CommonJS build at `dist/router.cjs.js`.
  * Update description.
  * Add version -- I chose `0.1.0` but perhaps it should be different.
  * Add `route-recognizer` and `rsvp` as dependencies.
* Add `.npmignore`, ignoring Ruby build files