Added Foursquare as a supported auth strategy.

[martindale]

This should fix #90. The following is the configuration I used:

    foursquare: {
      everyauth: {
          appId: conf.foursquare.secrets.clientId
        , appSecret: conf.foursquare.secrets.clientSecret
        , findOrCreateUser: function(sess, accessTok, accessTokSecret, foursquareUser) {
            var promise = this.Promise()
              , self = this;

            if (typeof(sess.auth) != 'undefined') {

              User.findById(sess.auth.userId, function (err, user) {
                if (err) return promise.fail(err);
                if (!user) {
                  return promise.fail('no user in twitter auth');
                } else {
                  assignFoursquareDataToUser(user, accessTok, accessTokSecret, foursquareUser);

                  // Save the new data to the user doc in the db
                  user.save( function (err, user) {
                    if (err) return promise.fail(err);
                    promise.fulfill(user);
                  });
                }
              });          
            } else {
              return promise.fail('no user');
            }

            return promise;
          }
        , redirectPath: '/'
      }
    }