allow send method not to call mrb_funcall if calling method is implemented in Ruby; fix #1680 ref #1765

Merge branch 'issues/comment_style' of https://github.com/cubicdaiya/mruby into cubicdaiya-issues/comment_style

remove unnecessary .gitkeep

According to CONTRIBUTING.md,

Don't use C++ style comments

 /* This is the prefered comment style */

Use C++ style comments only for temporary comment e.g. commenting out some code lines.

Please add me to the author.

mrb_fiber_yield() is available now; you have to link mruby-fiber mrbgem to use the function; there's no function available to create new fiber from C (countapart of Lua's lua_newthread), but that's because you cannot create a new fiber from C due to mruby C API design limitation. define your method to create fibers in Ruby; close #1269

Implement sprintf("%c") for UTF-8.

The string is "Hello world" in Japanese.  (hat tip:  Google Translate.)

fix SEGV bug for mrb_str_new_static

mrb_str_new_static causes seg-fault when 3rd argument is negative.