Add accepts_nested_attributes_for to commentable models

[danaustin]

[jackdempsey]

Hi Dan,

Thanks for the patch. My spidey sense is tingling though...it may just be the lack of opting in/out of this, or maybe some issues with ANAF...I'd feel more comfortable merging this in if it was possible for user's to opt into it. If you're confident this should be a default, then an opt out would be good, as well as your thoughts as to why it should be a default. I don't necessarily disagree but want to think on it a bit more with your opinion in there as well.

Thanks!