WIP autoproj versioning functionality

This pull request implements the support in autoproj for versioning, that is the ability to replicate an installation 1:1 without having to fear for changing software or package sets.

The following subcommands are planned:
 - creation of autoproj/overrides.d in which versioning files are saved
 - autoproj versions: generates a version file in which all packages are "pinned"
 - autoproj commit: generates a version file within the current autoproj configuration and commit it (assumes that autoproj/ is under git)
 - autoproj tag: generates a version file, and creates a commit+tag in the current autoproj/ git repository but leaves autoproj/ unchanged
 - auto-log: saves a log of all the updates done to the installation
 - autoproj log: displays the log
 - autoproj reset: resets the install to a known state from the log