Fix default return code; based @cubicdaiya #31 code
User mrb_irep_free() and initialize exc
remove return(refs #35 #36)

hello.mrb is used the example for mruby_content_handler.
But in this case, ngx_mruby does not return a response.
extend scope for handlers
Merge pull request #37 from cubicdaiya/issues/hello.mrb

remove return(refs #35 #36)
Merge pull request #38 from cubicdaiya/feature/extend_scope_for_handlers

extend context scope for handlers
DONE is already defined in mruby

 * mruby/src/init.c
Merge pull request #40 from cubicdaiya/issues/redefine_DONE

DONE is already defined in mruby
Merge pull request #42 from charlescui/master

fix bug #41. nginx sample conf is error.