v2.0.0: Serialization and primitive improvements.

This released introduces a new scheme for -list and -primitive types, whereby they are now allowed to extend other primitive classes per their definition in the FHIR specification.

This means that all -list type classes have moved out of the base namespace and into a sub-namespace under the name of the primitive they are extending. This is the primary reason that this is a major release rather than a minor.

There has also been work on changing how the primitive validation system works. Now, all properties on any type can have validation clauses. This work has not been finished yet, and I am targeting it for the next point release.

There has also been work on improving the json and xml serialization output, specially around how "value container" types are handled. Hopefully the current result is universally acceptable to implementors of this library.