Prepare for publishing to NPM

This PR is one step in addressing tildeio/router.js#69

It depends on https://github.com/tildeio/route-recognizer/pull/16; once this is merged in, I'll update the ref in package.json to point to the NPM module instead of the git repo.

* 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

For now, the `route-recognizer` dependency is pointing to my fork at https://github.com/spikebrehm/route-recognizer/tree/prepare-for-npm (see PR: https://github.com/tildeio/route-recognizer/pull/16).