Add the background selection

#9 

In the book permits the player and his GM to customise the equipments, feature, language, skills etc.

The bonds, flaw, ideals and traits suggestions I think we can add later on the personality screen.

For now, we'll assume the background provide fixed options.

- [x] Display the backgrounds.
- [x] Show the backgrounds details.

![backgrounds-page](https://cloud.githubusercontent.com/assets/867308/5160121/cb169b70-7368-11e4-8904-9d56b96c554e.png)


I think that here could be just `@background[:coin_pouch]` since it will be either `nil` or a hash with coins. If an empty hash is possible I'd prefer `empty?` here to keep more specific API requirement, what do you think?