Merge pull request #1 from mattn/win32

support windows.
Update README.md
add mrb_mruby_env_gem_final
add mrbgem test runner
update mrbgem.rake
update README.md
Use mrb_str_new_cstr
Merge pull request #3 from mattn/mrb_str_new2

Use mrb_str_new_cstr
Add stdlib.h
Merge pull request #4 from mattn/stdlib

Add stdlib.h (getenv(3) and others require it)
Fix build on VC
Fix mrb_raisef