patching for rails 4

[j-miyake]

• I replaced find_all_by_xxxxxx with where to let Rails4 projects can use this gem.
• I fixed the way to get the foreign keys to let custom foreign keys be used.

[rafaelrpbelo]

@johnny-miyake I see you are using reflection instead to constantize the key given.
Your patch enables us to use a through association with a custom class name?

[j-miyake]

Umm... I forgot why I made this pr, but maybe yes. As this repo seems to be no longer maintained, I'm closing this pr.