This library is being initially developed for an internal application at Google, so API methods will likely be implemented in the order that they are needed by that application. You can track the status of implementation in this Google spreadsheet. Eventually, I would like to cover the entire GitHub API, so contributions are of course always welcome. The calling pattern is pretty well established, so adding new methods is relatively straightforward.
Aside from the fact that Google is shipping open source on GitHub -- they're using Google Spreadsheets to track the status of this library's implementation.