Move read-only methods of 'Scoped' into new interface, 'ReadOnlyScoped'. (#195)

Not all subclasses of 'Scoped' can sanely implement 'with_scopes' (e.g, on
GCE the scopes are hard-wired in when creating the GCE node).

Make 'Scoped' derive from 'ReadOnlyScoped', adding the 'with_scopes' method.

Make GCE's 'credentials' class derive from 'ReadOnlyScoped'.

Closes #194.
4 files changed