Add new OAuth2 strategy, and deprecate old one

This is a mildly breaking change, and so therefore it revs the major
revision number for this package. Here's what needs to change:

```js
// Change the Strategy import:
var FamilySearchStrategy = require('passport-familysearch').Strategy;

// To LegacyStrategy:
var FamilySearchStrategy = require('passport-familysearch').LegacyStrategy;
```

A more involved change is needed to upgrade to the new
[OAuth2 strategy][familysearch-oauth2]. FamilySearch is [requesting that all
clients move to this new strategy][familysearch-auth].

[familysearch-oauth2]: https://familysearch.org/developers/docs/oauth2
[familysearch-auth]:   https://familysearch.org/developers/docs/authentication