Removing all instructor's guide material from the main repository

After several discussions, I've decided to take the instructors' guide out of the main repository:
1. It isn't ready for public consumption yet, so it doesn't really belong on the main web site.  (It will eventually be part of http://teaching.software-carpentry.org, but that's at least a couple of months away.)
2. Removing it simplifies (and speeds up) the build process.
3. Makes it clearer to newcomers that this *isn't* something they should be working on yet---I need to reorganize the structure (a lot) based on what we've learned in the last 15 months before it's ready for general hackery.