Improving Unix guide usability
Avoid suggesting higher Python version.
Move the note about Python 3 support.
Add Josh's suggestion.
Make it clearer that envvars must be updated
Fix some unescaped \'s
Fixing UNIX install doc re:ICE etc
Douglas' comments + Windows Ice clarification
Fixing Roger's comments
Merge pull request #707 from hflynn/rebased/develop/install

Updates to sysadmin installation docs (rebased onto develop)