- Moved MemoizedFDList to feature.py - Added AbstractFeatureClassifier, which factors out a bunch of common   code between NBClassifier and MaxentClassifier (now renamed to   ConditionalExponentialClassifier).
- Moved MemoizedFDList to feature.py
- Added AbstractFeatureClassifier, which factors out a bunch of common
  code between NBClassifier and MaxentClassifier (now renamed to
  ConditionalExponentialClassifier).


svn/trunk@418

nltk/nltk,nltk/nltk,nltk/nltk