Add init_db() call to example/tweet.py

[destan]

To launch the twitter example and use successfully, the db must be initialized.

https://github.com/mitsuhiko/flask-oauth/blob/master/example/tweet.py#L180

diff --git a/example/tweet.py b/example/tweet.py
index 536836a..2462345 100644
--- a/example/tweet.py
+++ b/example/tweet.py
@@ -178,4 +178,5 @@ def oauth_authorized(resp):


 if __name__ == '__main__':
+    init_db()
     app.run()