Adds functions for setting up recent bash version

Bash that ships with OSX doesn't support many nice modern Bash features
(e.g. associative arrays, auto_cd), because it's six years old (one
major version back: 3.2 vs 4.2).

So, if running the system bash, print out a warning message.  Then, if
so desired, the user can choose to use homebrew to install a better
version and set it up as the default shell.