Support upgrade and downgrade between versions

The "bin/RELNAME install" command now handles upgrading and downgrading.
It unpacks and installs new versions as needed, or just sets existing
releases as current/permanent as needed.

I renamed from "bin/RELNAME upgrade" because it handles downgrades too.
"upgrade" and "downgrade" are now synonyms for "install"

Additionally, when installing a new to-be-unpacked version, you can say:

  bin/RELNAME install 0.1.1/beat

OR just:

  bin/RELNAME install 0.1.1

in which case the /RELNAME part is added automatically. This is to keep
the version spec for downgrades the same as for new upgrades.

This is fully backwards compatible with the existing "upgrade" command.
Merge pull request #46 from RJ/create_RELEASES

Support upgrade and downgrade between versions