SchemaWalker for 1.0.2

[fge]

Enhancement to the core library, primarily motivated by two things:

• Is there a way to print a schema with references resolved? json-schema-validator#41 -- need to expand JSON References into schemas;
• generating other schema formats from JSON Schema -- for instance Avro.

The proposed code is build around three classes:

• PointerCollector: its implementations are dependent upon the schema version being used; an implementation collects relative pointers to be used by a
• SchemaWalker: dispatches subschema collections to PointerCollector instances, and when appropriate, calls upon a
• SchemaListener: hooks into a SchemaWalker at several points; these points are: walker initialization and exit, pushd/popd, walk of current node, schema tree substitution.

Integrating this into the core has required that ALL of syntax validation be moved in there as well -- but then I wanted to do this as well --, but also all the loading mechanism -- which I wanted to do as well.

Right now:

• the SimpleSchemaWalker, which does not resolve references, works OK;
• the RecursiveSchemaWalker works only in a limited subset of cases.

Immediate goal: make RecursiveSchemaWalker handle loops. RefResolver already does, but here it is more complicated than that: the latter doesn't need to be stateful, the former needs to be.

[fge]

Done, released.