Add multi-record support to attributes_table_for

Allow multiple records to be displayed in columns using the ActiveAdmin `attribute_table_for` component, while staying backward compatible with the previous behavior. It also builds a `colgroup` and `col` elements to facilitate styling each record's table column, but only when a collection is provided.

The PR is a result of discussions in #2540.

![attributes_table_for_collection](https://f.cloud.github.com/assets/25094/1286164/3e53d99a-2fc7-11e3-976a-0ba84e627969.png)