`Yesod.Auth.OAuth` now exports typeclass `YesodOAuth` ...

... and expects your foundation datatype to instantiate it.  Implementing this class means storing your Twitter app credentials in your site foundation instead of hard-coding them in `authPlugins`, as the previous version of this module required; you can load the credentials from environment variables, a config file, whatever.

Note: this first version of `YesodOAuth` only addresses Twitter auth; Tumblr auth works the same way it did before.  If I was going to use Tumblr auth in my own project, I'd make these changes there too.  I probably should have done that extra bit even though I'm not using it on my site, since the module now requires your foundation to instantiate `YesodOAuth` even if you're only planning to use Tumblr auth.