Make html_for_tweet() cope if a tweet has no entities.

I'm not 100% sure if it's possible any more to get tweet data that doesn't
include entities. I've a feeling it might be, but can't find an example.

However, `html_for_tweet()` currently almost handles a tweet not having
entities, except it throws `UnboundLocalError` because the `text`
variable is declared in the wrong place. This fixes that.

twython/api.py

@@ -544,8 +544,9 @@ class Twython(EndpointsMixin, object):
         if 'retweeted_status' in tweet:
             tweet = tweet['retweeted_status']
 
+        text = tweet['text']
+
         if 'entities' in tweet:
-            text = tweet['text']
             entities = tweet['entities']
 
             # Mentions