Move mrb_name_error() declaration to mruby.h As it is requred by struct mrbgem.
Use mrb_name_error() as possible.
Add `NameError#name`. Fix `NameError.new` and `NameError.initialize`. Enable 2nd argument for `NameError.new` in C API.
Add test for NameError#name
Add one more test for NameError#initialize. As we support NameError#name.
Merge pull request #1132 from monaka/pr-support-NameError.name

Support NameError#name