Removed the last vestiges of the MODE_MULTIPLE and Countable interface so
that we can integrate the new PropertySpecification/Property/Builder structure.
A separate parser/loader class will be responsible for loading and connecting
multiple (possibly interconnected) VCards, but getting it out of here
simplifies the API and the code.
Created a static registry of PropertySpecifications and defined
PropertySpecifications for every property in RFC6350.
Not connected to anything yet.