Add ability for campaign admin to enter a contact phone number

This adds the option for a campaign creator to share their phone number with backers.

If a phone number is provided, it is displayed in the confirmation email receipt as shown below:

![screen shot 2013-11-27 at 4 58 46 pm](https://f.cloud.github.com/assets/1132438/1636275/475ce6e8-57c8-11e3-8313-68db66177204.png)