This is for supporting json-based plugins. The great thing about this
change is that it allows us to easily print plugin version numbers in
`plugins list`.

All existing plugins are added to the binary bundle, in their latest
version, so that users don't need to pip install tutor.

Also, the tutor MANIFEST.in file was removed to simplify the management
of package data.

Close #242.

The k8s quickstart command is now functional, with suppport for https,
xqueue, notes and minio. There are still a few bugs to get rid of,
though.