Portability / resource mangement

indian.py got rid of codecs to make it more portable
megam.py got rid of import os.path and import os, not needed and it increases stability without it
cleaned up imports for pt.py to make code more portable and more efficient
simplified code in relextract.py import nltk.sem import relextract is recusive
string_category.py removed import os and general from nltk import compat to make it run with less resources

Where is nose being defined?
nltk/setup.py What setup tools that are to be imported?
Wordfreq.py where is pylab coming from?

could you please move API changes to another PR?