updated contributors
encoding option for doctest files in runtests.py
use nltk_data or nltk_data_3 as the directory name for data files depending on the current python version
use http://nltk.github.com/nltk_data/ or http://nltk.github.com/nltk_data_3/ as the download url depending on the current python version
reuse code from nltk.data to get the default download path, if no existing nltk data directories are present