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
Anything that the install process prints to stderr (as opposed to stdout) gets colored red. (Good thing regular terminals don't do this, heh...)

The note here is good but I'm trying to avoid hard line breaks in this file; can you re-wrap? (the graf in 25-27 should be on one line)