Switch user profile from xml to json. Improve example app.
Fixups for express3.
[API break] Retire Strategy to LegacyStrategy.

Since it represents the old way of authenticating with FamilySearch,
we retire the old strategy in a way that is minimally breaking, but
it does cause us to rev our major version number.
Move the files around in prep for new Strategy.
Add new OAuth2-based authentication strategy.

It also uses the new /platform API for requesting current user profile info.
Update README to show new and legacy usage.
sessionKey option not needed in OAuth 2.0-based strategies.