Improve the contents of doc/samples

* Remove the "Izvestia" articles (as being quite different in their topics)
* Remove the keywords used just for testing before
* Add keywords for economic processes
Change the default score threshold

The score gets moved up from 0.2 to 0.25 (after conducting several experiments)
More subtle words filtering in synonym extractor

The synonym extractor filters out words that occur too rarely in the text
collection. Here, we make "rarely" not to mean to be smaller than some
constant frequency, but dependent from the overall text collection size.
New doc/samples

Here we add excerpts from the rules of National Research University -
Higher School of Economics to the doc/sample folder, as well as a
set of keyphrases associated with it.

This set of texts produces a quite nice output while being analysed
with the corresponding keyphrases (keyphrases graph construction).
It not only makes sense but also differs a bit when turning on
synonym extraction, thus proving the significance of this process.