Added python file for parsing corpus.
Added corpus and query files. Implemented CorpusParser.
Changed name.
Added class for parsing query file.
Implemented InvertedIndex (untested). Implemented table for doc lengths (untested).
Refactored doc parameters to docid.
author
Raise exceptions if keys not present.
Added function for finding how many documents contain a word.
InvertedIndex debugged and fixed. Added function for retrieving average document length from document length table.
Implemented ranking function. May  need refactoring.
Added test cases.