Add new OAuth2-based authentication strategy

We're asking all developers to move to [our new OAuth 2.0 authentication strategy](https://familysearch.org/developers/docs/oauth2). We're [actively discouraging](https://familysearch.org/developers/docs/authentication) the continued use of the old OAuth 1.0a approach, and as such we've made a very small, but API-breaking change to move that strategy out of the way and make it obvious that it is much less preferred.

The changes in usage are minor, and (except for the `profile` object changes) they are detailed in the README.md update. As these changes _are_ API-breaking, we have revved the major version number in keeping with [semver](http://semver.org/). We've added additional tests for this new `Strategy`, and renamed the previous files as needed to reflect they focus on the legacy API.

We've also lightened up the dependencies needed by taking the change proposed by Logan Allred in #1 to use the pure JSON API method, so if you pull this change you shouldn't need to pull the other. That change removes the need for `xml2js`.

Finally, we've also updated the `profile` object to use the new [FamilySearch Platform API](https://familysearch.org/developers/docs), and ensured that the data in the profile object matches what's available using that [new API](https://familysearch.org/platform/). Naturally, along with this change, we've added another fully-working example similar to the existing one.