Merge pull request #116 from wanabe/mruby-accessor

Wrap member 'next' of grn_accessor in ruby method

Patch by wanabe. Thanks!!!
Merge pull request #117 from wanabe/mruby-grn_scan_info_set_op-and-end

Wrap grn_scan_info_set_op() and grn_scan_info_set_end() in ruby method

Patch by wanabe. Thanks!!!
mruby: Move grn_accessor related codes in a new file