Implement "scopes" #42

It would be nice if we could have different sets of dependencies for different uses, the typical would be for compilation, runtime, testing, etc.

We'd have to figure out how to:

• add this to the current app.yml format without making too many changes
• be sure that the app.yml format stays fully backward compatible
• design new format making it flexible enough but still easy to use
• how to store things on disk (deps/compile, deps/run?)
• add a cli option for applying scope to a command? (eg jpm install -s compile foo:bar:123?)
• add extra options to the interactive search for dealing with scopes? (eg "Install to scope" and "Copy to scope"?)