stemming a text file into stemmed contents list or output text file

nlp.stem_text can stem a text file into stemmed contents list or output
text file.

Also, I changed examples/stem_text.py to use nlp.stem_text and it has
been more simple to use.

usage:
$ python stem_text.py [stemmer_name] [input_file] [output_file]

example:
$ python stem_text.py lancaster data/text/description.com.nytimes.an
droid.com ouput.txt