Removed class ``Word`` - word is simply string
Better algorithm for removing trailing zeros

Old algorithm was slow because for large data
copied items to mutable list firstly.
Added headings property to DOM elements

Property 'sentences' contains only common sententes
and property 'headings' contains headings.
Properties are cached.
Refactored Edmundson's Cue method
Refactored Edmundson's Key method
Refactored Edmundson's Title method
Refactored Edmundson's Location method