make search-index module browserifiable

Plan:
- [x] switch dependency on `level` to `levelup` and `leveldown`.
- [x] add [`level.js`](https://www.npmjs.com/package/level-js) for use in browser
- [x] accept a `db` property in the `SearchIndex` options.
- [x] add `search-index-browser.js` that exports same API as `search-index`, but creating and passing in a `level.js` db.
- [ ] set up the test suite to be runnable in the browser (don't know if PhantomJS has IndexedDB, or if Travis can handle other browser tests...)