add check for twython import

nltk/twitter/__init__.py

@@ -13,6 +13,14 @@ This package contains classes for retrieving Tweet documents using the
 Twitter API.
 
 """
+try:
+    from twython import Twython, TwythonStreamer
+except ImportError as err:
+    import textwrap
+    MSG = """The NLTK twitterclient module requires the Twython package. See\
+    https://twython.readthedocs.org/ for installation instructions."""
+    err.msg = textwrap.fill(MSG)
+    raise
 
 from nltk.twitter.util import credsfromfile
 from nltk.twitter.twitterclient import Streamer, Query, Twitter, TweetViewer,\