Custom keyserver and apt-repository attributes

Hi there,

Thanks for sharing your cookbook.
Since I'm behind a firewall, I was unable to successfully run the apt_repository recipe because the connection to the keyserver on the default port is blocked. I took the liberty to add some attributes to allow a custom keyserver and apt-repository. The readme is updated including an additional note about this possible firewall issue.

I thought you might find it useful, so feel free to pull.