title: nltk doesn't add $NLTK_DATA to search path?
body: <p>under linux,I have set env var $NLTK_DATA('/home/user/data/nltk'),and blew test works as expected</p>
  
  <pre><code>&gt;&gt;&gt; from nltk.corpus import brown
  &gt;&gt;&gt; brown.words()
  ['The', 'Fulton', 'County', 'Grand', 'Jury', 'said', ...]
  </code></pre>
  
  <p>but when running another python script,I got:</p>
  
  <pre><code>LookupError: 
  **********************************************************************
  Resource u'tokenizers/punkt/english.pickle' not found.  Please
  use the NLTK Downloader to obtain the resource:  &gt;&gt;&gt;
  nltk.download()
  Searched in:
  - '/home/user/nltk_data'
  - '/usr/share/nltk_data'
  - '/usr/local/share/nltk_data'
  - '/usr/lib/nltk_data'
  - '/usr/local/lib/nltk_data'
  - u''
  </code></pre>
  
  <hr>
  
  <p>As we can see,nltk doesn't add $NLTK_DATA to search path,after appending NLTK_DATA dir manually:</p>
  
  <pre><code>nltk.data.path.append("/NLTK_DATA_DIR");
  </code></pre>
  
  <p>script runs as expected,question is:</p>
  
  <p>How to make nltk to add $NLTK_DATA to it's search path automatically?</p>