Expand on shell portability concerns

- Explain the difference between /bin/bash, /bin/sh, and /usr/bin/env bash when specifying an interpreter for a shell script.
Wrap string literals per review comments,
Explain what the hashbang is and why it's important
Make bash manual a link, rather than have the link floating off in space.
Attempt to address outstanding review comments.  (typos hopefully all fixed now).
Merge branch 'master' of https://github.com/sdaugherty/curriculum into sdaugherty-master
programming_101: typo fixes, term creation for glossary