GitHub:Enterprise support

We've got a GitHub:Enterprise instance and it'd be neat to be able to use DCIY with it. I think it makes sense to support Enterprise, especially since DCIY is mostly useful for non-public repositories.

If an `ENTERPRISE_HOST` environment variable is set, DCIY will give you a dropdown for the hostname when you're creating or editing a `Project`, letting you choose between the enterprise host, github.com, or whatever the project is already using (so it doesn't get all weird if you change the setting around):

![dciy-enterprise-dropdown](https://f.cloud.github.com/assets/17565/1860420/171728d0-77b3-11e3-8921-dc5be91bf1de.png)