Handle private disabled repos

I'm putting this PR up partly to remove a personal annoyance, and partly for discussion.

I used to have a paid GitHub account containing two private repos. When I decided to stop paying for GitHub, the repos became disabled, but are still in my list of starred repos. As a result, every time Trailer checks for new repos, they get detected, then subsequently nuked, because the URL for pull requests responds with a 404.

I've made a change her that adds a private flag (name up for debate of course) to the Repo entity, and sets that from the "private" key in the repo dictionary received from the API. After the URL for pull requests is accessed, if there is a 404 received for a private repo, I've set the `postSyncAction` to `kPostSyncDoNothing` rather than `kPostSyncDelete`.

There are unanswered use cases - what if the repo is no longer disabled (e.g I decide to pay for GitHub again) - will it check again for PRs against these repos or not.

What do you folks think?