Remove notice of early version
Add download link for stable version to README

- add download link of stable version 1.0.0
- remove remark for mruby branch due to non-existence
rename mrb_name_class and make it static
made mrb_define_class to return existing class, with heavy refactoring
Merge pull request #1690 from bovi/patch-2

Add download link for stable version to README
Merge pull request #1689 from Bovi-Li/remove-notice

Remove *very early version* notice
forget to add function prototypes