setup: move version string to zpmlib
doc: use version number from zpmlib
version: new command for showing version number (#103)

This comes with a little test that should remain stable as the version
number changes in the future.
zpm: add support for --version flag (fixes #103)

This uses the special 'version' action in argparse. In Python 2.7 and
later, argparse automatically supplies a suitable help text, but not
in Python 2.6. We therefore specify the help text ourselves.
Merge pull request #107 from mgeisler/version

Add version command and --version flag