Add bin/buildout --version

Currently to see which version of zc.buildout I've got, I have to open bin/buildout in a text editor and look at the version number in the filename of the buildout egg included in sys.path. I'd like a simpler way. Having the --version command line argument print the version number and exit without doing anything else is traditional.

Fixes https://bugs.launchpad.net/zc.buildout/+bug/309770