GitHub API should expose the source used for GitHub Pages

[stuartpb]

Now that a repo's GitHub Pages source is semi-configurable (see also #212), the information about which branch and directory is the source for GitHub pages can't merely be deduced from the repository's name (previously it was guaranteed to be the root of master if the repo was named <owner>.github.io and the root of gh-pages otherwise). It should be exposed as part of the GitHub Pages API for repos - both as part of the repo's base information, and as part of the information for each build (so the point where the builds' base was changed can be detected via API).