add ppa support to pkgrepo

Ubuntu has a Launchpad site that supports Personal Package Archives.
This is where the newest (untested, unsupported) software can be found,
including backports of newer versions of packages to older releases.

This branch supports a ppa option that makes it a one-liner to add
a ppa to your apt sources.
Merge pull request #4021 from ahammond/add_ppa_support_to_pkgrepo

looks good.  pulled & tested briefly.. let's do this!