fixed autoproj snapshot args checking

expand_path does not like given nil, which leads to an error before the
actual error message can be displayed.
change success/error message

this is to reflect that an autoproj command does not necessarily
need to be a build command.
added a version method to the relevant importers

before, the snapshot functionality was used, but this is often too much,
just to recover the version of the package. Changed the snapshot to use
the version method.
added versions method to cmdline

similar to snapshot, this will return a list of packages and their
respective version information. Unlike snapshot it does not actually
snapshot any files.
first version of autoproj-commit

mainly for testing
Extended package_set to support multiple override files

The files need to be placed in an overrides folder. This only applies to
the LocalPackageSet. The old overrides.yml is still used at the end
of the overrides chain.