2 2 dev promo categories

This is a 2.2 compatible version of https://github.com/spree/spree/pull/4992  One minor change was required for compatibility in commit adbdb0a, otherwise everything works correctly.

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This allows a promotion to be categorized on the administration interface for easy sorting. It helps an administrator in sorting through promotions, which can be a bit unwieldy. This has no effect on promotions from the front end.

Examples of usage:

### Apply a filter on the search page.
![screenshot from 2014-07-16 15 36 15](https://cloud.githubusercontent.com/assets/764390/3606673/cd5f6258-0d39-11e4-95ff-7f483df1c9d6.png)


### You can click the link to manage promotions to be taken to a basic CRUD for managing promotion categories.
![screenshot from 2014-07-16 15 36 29](https://cloud.githubusercontent.com/assets/764390/3606674/cd5fcc98-0d39-11e4-8ed6-4b85ab6bfce2.png)


### The individual promotion page will allow you to select the promotion category for this promotion.
![screenshot from 2014-07-16 15 37 05](https://cloud.githubusercontent.com/assets/764390/3606675/cd61430c-0d39-11e4-80ac-3764a732458d.png)