Add Nginx#errlogger

example:
Nginx.errlogger Nginx::NGX_LOG_ERR, "ngx_mruby error!"
Add Nginx#nginx_version and Nginx#ngx_mruby_version

example:
        Nginx.rputs "ngx_mruby_version: #{Nginx.ngx_mruby_version}<br>"
        Nginx.rputs "nginx_version: #{Nginx.nginx_version}<br>"
fix problem comparison between signed and unsigned value

If Nginx.errlogger recives 1st args as negative number,
ngx_log_error works incorrectly.
Merge pull request #16 from cubicdaiya/issues/error_level_type

comparison between signed and unsigned value
fix typo
Merge pull request #17 from cubicdaiya/issues/fix_typo

fix typo