Move SSH Key lesson from Git to Extras

Related with #691.
On Wed, Sep 10, 2014 at 05:42:18PM -0700, r-gaia-cs wrote:
> @wking thanks for the remember but I believe students already have
> too much information and because of it I prefer to skip this extra
> information.

I'm fine with skipping information, but SSH keys won't necessarily
save you any time on usernames/passwords.  I'm not sure if GitHub's
HTTPS authentication is basic or digest [1], but I suspect it's basic
[2].  SSH is just better because you don't have to worry that maybe
your password is going across the wire; with SSH the private key
always stays local.  I've also never written a 4kbit password ;).  So
the benefits to SSH are:

* Stronger secret
* You don't share your secret with GitHub
* You can use the same secret safely in many situations (e.g. I use
  the same SSH key for GitHub as I do for logging into my work boxes,
  but that would be a bad idea with passwords, because a compromise at
  either location could expose the password I use at the other).
* Reversing a salted and hashed password is hard, but probably still
  easier than finding the private key matching a given public key.

For the purpose of this lesson, I'd just stick to the “additional
configuration” argument, and drop:

  “Unfortunately this option also requires you to type your username
  and password every time you interact with Git remote server.”

Then have:

  “After the bootcamp you may want to set up SSH access, which is a
  bit more secure, by following…”

[1]: http://tools.ietf.org/html/rfc2617
[2]: https://developer.github.com/v3/auth/#basic-authentication