add mock locale and method swizzling to make test for issue #15 fail

the swizzling allow us to return a special configured locale in `[NSLocale currentLocale]`. `+currentLocale` is used in NSDateFormatters that don't have an explicit locale.
the mock locale uses 24 hour mode and has the private flag for "force 12 hour mode" activated
set locale of unparsing NSDateFormatter to "en_US_POSIX" to fix issue #15
Merge pull request #17 from mattbauch/master

Made test for #15 fail and fix issue #15