change archetype schema accessor and mutator

@rpatterson hey, i've changed the getter and setter methods for schemaupdater.
Coul'd you take a look on it and merge my branch into the master if it's ok?

Fix field accessor and mutator for updating schemaextended field values with schemaupdater. In some cases when using fields extended by schemaextender it defines an accessor attribute which is not accessable. To cover all fields, its better to access and mutate over the getAccessor and getMutator methods on archetype fields.