Add ISO date formatter stub, wants expansion

There now is a stub class `ISODateFormatter` that can be used as a singleton and should end up parsing a slew of valid ISO dates.
Improved unit testing

Unit test classes are now read from disk, relative to the source file location. This way the example JSON no longer have to be added to the Xcode project. The path assumes the location given by the "SwiftFHIR" project.