Eventually I will implement such things in Magit (I am the maintainer), but I first have to get a release out.
Old outdated information: Unfortunately there currently also isn't a third-party extensions that could fill this role. magithub has been broken for a long time now. magit-gh-pulls (by Yann, my predecessor as Magit maintainer) also wasn't kept in sync with the changes in Magit. I tried to fix it up some time ago, but gave up when it became clear that doing so would result in a complete rewrite . gh.el is also written by Yann and is used by magit-gh-pulls. I have contributed to it in the past but eventually stopped using it because (a) it uses url.el and that turned out to be very unreliable (b) it's over complex.
So I am afraid there currently isn't a package that does what you want. If you want to write it yourself, I recommend you use request.el and then only implement those parts of the Github api that you actually need, to avoid over-engineering it.
Edit: As of October 2015 magit-gh-pulls is maintained again, but no longer an official extension. Personally I don't use it, as I think it should either to more or less. I currently use "less" in the form of magit-branch-pull-request from the magit-rockstar library. Even though I maintain that library, I do not consider it an official extension - it's provided as-is. That function is very basic, you give it an issue number and it creates a branch for you, that's it.
Edit in September 2016: I have written ghub.el and glab.el as replacements for gh.el. They are mostly intended for my own personal use and provide very little, basically they give you functions such as ghub-get (resource &optional params data noerror), you then have to look at the respective api documentation to figure out what resource, params, and data you have to use. Also the error handling isn't great and this currently uses url.el. I intend to improve both eventually by using libcurl some time after an Emacs with ffi support has been released.
Edit in January 2020: More than a year ago I have released forge.
Forge allows you to work with Git forges—such as Github and Gitlab—from the comfort of Magit and the rest of Emacs.
call-processandset-process-sentineland shell-out behind the scenes so that you never feel like you are leaving the current buffer -- the libraries you mentioned are shelling-out anyway. If the function you want doesn't exist, then just create it like the unrelated function in the following link (which pushes to multiple repositories): https://mcmap.net/q/740624/-emacs-how-to-push-a-git-repository-to-multiple-remotes – Ardell