Add Swift Package Manager configuration (#30)

* Add Swift Package Manager config

* Update Package config OAuth2 release

* Update Package config OAuth2 release

Merge pull request #28 from smart-on-fhir/develop

Update to FHIR R4 and Swift 4.2

Fix compiler warnings, update submodules

Swift 3.1 fixes

Version 3 for Swift 3 and FHIR STU-3

Final days of DSTU-2