don't use static class instances
Merge branch 'master' of https://github.com/mruby/mruby
Merge branch 'master' of https://github.com/mruby/mruby
Merge branch 'master' of https://github.com/mruby/mruby
Merge branch 'master' of https://github.com/mruby/mruby
remove compiler warnings.
Merge pull request #718 from mattn/remove_compiler_warnings

Remove compiler warnings