revamped yaml headers and other things for exersise knitting.

* Moved things so students are less likely to accidentally edit them.
* Changed YAML header in trial_homework so that
default is to render to HTML with a TOC unless
specifically asking to do it with bookdown.
* Add gitignore to keep .html files from the exercises
directory out of the repository.
* Added line in build-site.sh to remove .html files
that might be left over from testing rmd's before jekyll builds it.