Rename SimpleHTMLParser.py to html_parser.py
Rename IndexLoader.py to index_loader.py
Rename SimpleSearchEngine.py to search_engine.py
[crawler.py] Fix ignoring os exception when unable to create ./html/ dir
[preprocessor.py] Fix ignoring os exception when unable to create ./tokenized/ dir
[preprocessor.py] Accept arbitrary dir containing '.html' files

preprocessor.py now recognizes a dir name as a command line argument,
and will try to clean and tokenize all its '.html' files.
Add usage section to README