Added option to collect shipping for only specific rewards

Introduces the ability to collect contributor shipping addresses for specific rewards, as opposed to only being able to collect shipping addresses for all rewards or no rewards.

Addresses issue #152 

![screen shot 2013-12-27 at 3 16 02 pm](https://f.cloud.github.com/assets/1132438/1816770/cd22e62c-6f4e-11e3-8ab9-c22210ddf9ff.png)