Handle subprocesses consistently

[ra-tolson]

gips uses two or three ways to call subprocesses, and the handling isn't consistent. Write a subprocess handler that does things in a unifed way almost like we have for error handling. Stuff we could do in the handler for consistency:

• Choose a single standard library or third-party thing that does what we want, and use it everywhere.
• Raise an exception with a predictible type and content on nonzero exit status, or even optionally wrap in a with error_handler if the caller wishes.
• Consistent printing of the subprocess's invocation line on -v4 or -v5.
• Options to handle the process's stdout and stderr in different ways (pipe to /dev/null, save to a file, show to the user).
• Options to buffer and unbuffer the process's streams (I think this is helpful when monitoring a long-running process that's mostly logging to a file, which is buffered by default).

For the initial implementation, these basic features should be sufficient, plus a few conversions of existing code like we did for error handling. Then future iterations can convert all the subprocesses in gips, and improve the implementation as we see fit.

[ra-tolson]

Regarding which lib to use, commands has a deprecation warning, and a claim about subprocess having better features; subprocess says you should use the backported version from python3:

https://pypi.python.org/pypi/subprocess32/