convenience method for defining active record scope identifiers

We can now define ActiveRecord scope identifiers, along with accounts, thusly:

```ruby
  DoubleEntry.configure do |config|
    config.define_accounts do |accounts|
      user_scope = accounts.active_record_scope_identifier(User)
      accounts.define(:identifier => :checking, :scope_identifier => user_scope)
    end
  end
```

This fixes #40.