Added arbitrary package attributes

You can now include any arbitrary attribute in a package.py. It will remain in the package definition on install/release (previously it was stripped), and can be accessed like 'Package.mycustomattrib'.

Motivation: The need to perform custom post-package-release actions depending on certain attributes. For example, I have a post process that generates documentation on a server, but I don't want to do that for 3rd party software, and in those cases I will add an "external = True" attribute to the package.py.