Apt-select

* Adds support for Ubuntu minimal rootfs (core docker images).
* Improves system information processing, error handling on non Ubuntu systems.

* Fixes 62 to allow an optional `--country` argument.
Defaults to `US` (breaking change).
* Send `User-Agent` header in requests to respect Launchpad's
[request regarding automated querying](https://help.launchpad.net/TermsofUse).
* Fixes 59 to support replacing https mirrors in `sources.list`.

Package and library updates.

- Fix 49
- Improve code quality
- Add column spacing for `--ping-only` output
- Improve output wording/formatting

- Removed an errant version test debug line being printed to the screen.

PyPI support with installation via pip.