Test the abstract generated by the CACM parser.
Separate Document into CacmDocument and RequestDocument.

Will enable us to add WikipediaDocument later on.
Merge pull request #8 from Neki/topic/bf_better_document_structure

Make specific classes for CacmDocument, RequestDocument...
new classes : DocInfo and InvertedIndex
documentation + fix for research + pass
add stats to DocInfo
normalize_weight method & test ok
similarity & sort
new inverted index