Several changes has been made:
 - Parallel result merge
 - Scoring algorithm slightly changed (see result_score())
 - Proper Thread locking on global data manipulation

2 = strict
1 = moderate
0 = none

Allow to perform a search while adding an engine (or a category) without adding it "officially" to the request.
'?' is used to add an engine without modifying anything else to the request. For example, you can perform a search in the 'general' category, and if you add '?tw' the result from Twitter will be added to the originals results.

Lots of Flake8 corrections
Maybe we should change the rule to allow lines of 120 chars. It seems more usable.

Big twitter correction : now it outputs the words in right order...

thanks @Cqoicebordel in #144:
https://github.com/asciimoo/searx/pull/144#issuecomment-67036903

* add comments
* add licence-header