Significantly improve ranking:

1) rank_cd now use weight of lexemes
2) rank_cd and rank can use any combination of normalization methods:
        no normalization
        normalization by log(length of document)
        -----/------- by length of document
        -----/------- by number of unique word in document
        -----/------- by log(number of unique word in document)
        -----/------- by number of covers (only rank_cd)

Improve cover's search.

TODO: changes in documentation