basic implementation of polygon format loader

Polygon (https://polygon.codeforces.com) is a great tool for preparing programming contests. It's widely used on http://codeforces.com, in Russia, Kazakhstan and in other countries.

This loader allows to import prebuilt contest package from polygon. I've used it on our contest and I think that it can be useful for others CMS users.

Polygon uses testlib.h (https://code.google.com/p/testlib/) for checkers, and it's format is not compatible with CMS. I included modified version of testlib.h, so Polygon checkers can be compiled to use in CMS environment.