Add Andrew Moffat's sh.py library (MIT license)

We'll be using sh.py as a replacement for the subprocess module in the
standard library. Rather than mess around with asking the end users of
Rapuma to install it on their systems, since sh.py is a one-file library,
we'll just install it alongside the Rapuma libraries.
Merge branch 'master' of https://github.com/sillsdev/rapuma