jsonalchemy: update readers and SmartJson

• Update all parsers to work with the new structutre of the field and model definitions, meaning using the extensions properly.

• When translating a new blob there is no need any more to go through the readers registry, instead call Reader.translate. See the method docs for more info.

• Update SmartJson to work also with new extensions and decorators structure.

• Update JSON for marc producer to adapt it to the new extension behavior.

• Add first version of sphinx documentation (to be completed).

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>