Allow connecting to Battle.net via OAuth

[cheshire137]

Use Blizzard's OAuth API to allow a registered user to connect their account with Battle.net, or to allow an unregistered user to register their account via signing in with Battle.net.

I'm assuming we can get the user's Battle.net ID from this API somehow. We should verify that. If it's possible, we should auto-fill the users.battlenet field #30 with it. We should also then update any players records that have that battlenet so that their user_id is that user's.

https://github.com/Blizzard/omniauth-bnet looks like a good gem to use with Devise; see https://github.com/plataformatec/devise/wiki/OmniAuth:-Overview.