Swift 3 and FHIR STU-3 Freeze Jan 2017 (v1.8)

Workarounds for Linux

This disables FHIRAbstractResource’s `instantiate()` method because of a compiler crash (argument type mismatch when it calls out to the factory). Also adds a somewhat nasty conversion from `Decimal` to `Double` – needed while NSJSONSerialization works differently on Linux and macOS.