swc-installation-test-1.py: Give suggested install hints

This can link to the "howto setup Python" notes from SWC once those
get a more precise link than:

  http://software-carpentry.org/setup/
swc-installation-test-2.py: Give suggested install hints

I can't give package-specific links yet, but they will hopefully be in
place soon.  I made the "email your instructor" suggestion optional,
because it is easy for me to imagine a course large enough that the
instructor could not field all requests.  To disable, change

  print_suggestions(instructor_fallback=True)

to

  print_suggestions(instructor_fallback=False)
swc-installation-test-2.py: Break after first successful or-dependency

No need to test the other dependencies once you find one that matches.
swc-installation-test-2.py: Sort shells in order of preference

Placing the often-symlinked `sh` at the end of the list.  This way if
a user has Bash installed, it doesn't show up as plain old Bourne.
swc-installation-test-1.py: Give instructions for 'python command not found'