add dataset parameter to fetch_20newsgroups()

As some literature papers use different versions of the 20ng dataset, this parameter will allow users choose which version of the 20ng dataset to download:
1. 'bydate' --> download the sorted by date version (18846 documents) 
@ http://people.csail.mit.edu/jrennie/20Newsgroups/20news-bydate.tar.gz

2. 'original' --> download the original version (19997 documents) 
@ http://people.csail.mit.edu/jrennie/20Newsgroups/20news-19997.tar.gz

3. 'preprocessed' --> download a preprocessed version where duplicates are removed and only "From" and "Subject" headers exist (18828 documents) 
@ http://people.csail.mit.edu/jrennie/20Newsgroups/20news-18828.tar.gz