First version of glossary.

1.  Created a tool `bin/gloss.py` and a Makefile target `make gloss` to check glossary term uses and definitions.
2.  Filled in glossary terms used so far in the `novice` material.
3.  Fixed errors and omissions detected while doing 1 and 2 (e.g., we were copying `./tmp` to the `_site` directory while building).