Improvements and bug fixes

Bug fixes :

• Fix reindex vector on argument error #470 (by @Yuki-Inoue)
• DataFrame#set_index can take column name array, which results in multi-index #471 (by @Yuki-Inoue)
• implements DataFrame#reset_index #473 (by @Yuki-Inoue)
• Make DataFrame.from_activerecord faster #464 (by @paisible-wanderer )
• Optimize aggregation #464 (by @paisible-wanderer)
• Added access_row_tuples_by_indexs method #463 (by @Prakriti-nith )
• Index#dup should copy reference to name too #477 (by @Yuki-Inoue)
• Should support bundler version 2.x.x #483 (by @Shekharrajak )
• fix table style #489 (by @kojix2 )