Depend on json-schema-validator 2.0.0

And remove dependency on json-schema-core since this dependency is provided by
-validator.
Add two utility classes

One is a simple implementation of ValueHolder. The other is a class allowing to
inject and retrieve raw values out of a Processor whose inputs and outputs
implement ValueHolder.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>