decorator @py3_data for use with the file pointers in data.py
* for use on the initializers, e.g. BufferedGzipFile
* adds PY3/ to the data path for the following datasets:

  chunkers/maxent_ne_chunker
  help/tagsets
  taggers/maxent_treebank_pos_tagger
  tokenizers/punkt
apply @py3_data decorator to the path pointer init methods, to add PY3 to paths for selected corpora
Merge pull request #470 from stevenbird/master

decorator @py3_data for use with the file pointers in data.py