an attempt to fix issue #111

`boost::locale::to_lower()` correctly transform string to lower case, but it requires to set global locale (or pass locale in second argument). To set locale `std::locale::global(boost::locale::generator().generate(""))` is used, and it requires libboost_locale to be added to makefile.

PS. I have some errors trying to build twister in cygwin, so the code is UNTESTED! Please, test it before merge.

PPS. Maybe there exist a way to set utf-8 locale in Windows without dependence on libboost_locale, but I don't found it (seems the only way is to create custom `Facet` class).