Add in an active_record_helper

Please consider this pull request as the start of a discussion about if this file is needed.

Feedback welcome.

Thanks.
Thanks for the feedback Jon. 

Can I assume that "not everything" means that there are some database adapters that don't support transactions? I would have thought that ActiveRecord would have abstracted away this issue but it sounds like I am mistaken.

Is this wide spread enough to roll this out and fix iteratively?

I agree that if it would cause widespread issues, or issues for people who might have issues quickly fixing the problem, then adding it in would not be a good idea.

If it wouldn't be very widespread, or those that would have issues would be easily able to fix them, then adding it in and fixing it iteratively might not be a bad idea if the team feels like the idea is worth it.

I am also OK dropping the idea all together if people think that it is too out of line with the direction of rspec-rails.

Thanks again for your time.