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
No, this should be "simple" (see http://git-scm.com/docs/git-config), that's the new default.

Trusty it still on git 1.9.1; the comment should say that it's to match the default behavior of git 2.0 (which is what students will get if they later install it directly/with homebrew/from backports).