#2 - Crichton refactoring descriptor element

@fosdev @chad-medi @sheavalentine-mdsol @jcarres-mdsol 
Added descriptor_element class which represents descriptor. Used to dereference itself and build dereferenced_hash registry - will contain all dereferenced descriptors from all descriptor_documents loaded by crichton.

This must be merged after #146 

Please review and merge.