add --version argument

[Karl-Philipp Richter]

It would be helpful to have a `--version` parameter for the command printing the current version of the binary and of the versions of the most important dependencies if that makes sense.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: qtqr 1.4~bzr21-1
Uname: Linux 4.2.5-040205-generic x86_64
ApportVersion: 2.19.1-0ubuntu4
Architecture: amd64
Date: Sun Nov 1 18:48:17 2015
InstallationDate: Installed on 2015-09-14 (47 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: qr-tools
SystemImageInfo: Error: command ['system-image-cli', '-i'] failed with exit code 2:
UpgradeStatus: Upgraded to wily on 2015-10-23 (9 days ago)