$N Multistroke recognizer

This is for discussion. Me and @bionoid will be committing here. It would be great if @tito could point out what's missing.

Current TODO:

* remove Golden section search. Protractor is faster and more accurate;
* refactor multistroke demo making it Kivy compliant.

Changes I made after the old pull request:

* `multistroke.py` PEP 8 refactor;
* `touchanalyzer.py` -> `kivy.uix` (as discussed before);
* replaced `GPoint` with kivy's `Vector`;
* minor changes to the demo.