JsonLoader: change classpath resource loading

For this, use Guava's Resources package to obtain the URL.

However this doesn't use the same scheme as .getResourceAsStream() directly from
a class; it uses a ClassLoader. So, replace the initial / (if any) with nothing.

This is a change in behaviour so it will have to be well documented...
JsonLoader: document the new behaviour

I don't really like to do that, but here there seems not to be the choice.