Remove a bunch of scary warnings around Git

* Set name and email in gitconfig so students can actually `git commit`; otherwise they can't commit at all
* Automatically trust Heroku's SSH key, to prevent a warning about "The authenticity of the host can't be established"
* Set `push.default = current` in gitconfig so `git push heroku` (vs `git push heroku master`) works without an error
Just put this in a dotfile instead.