Use mrb_str_cat() instead of mrb_str_cat2() as possible.
Rename API mrb_str_cat2() to mrb_str_cat_cstr(). This is for naming orthogonality. mrb_str_cat2() is also left for backward compatibility.
Merge pull request #1050 from monaka/pr-cleanup-string.c-20130323

Clean up string.c