Several minor changes

- Converted html to text in retail_math_taxons.txt source file
- Added config option for page relevance confidence threshold
- Changed the defaults for the number of analyzed pages
- Set the weight function for search results to be 1/sqrt(k),
where k is the result's position in the return list (after some
experimentation - no matter how many pages are analyzed, the proportion
of weights for the first k results is always the same; on the other
hand, the function is decreasing but not too steeply, hence second
result is not considered fundamentally less important than the first one)