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.
This will wrap every spec in a transaction regardless of wether it touches the database, this is normally a bad thing. This should be attached to spec types that are considered likely to touch the database. e.g. `controller` `model` etc. Even if that's all the `rspec-rails` types.