[Swift] Adding Swift3 generator

* This address #3788
* Contains all changes made in #3823
* Also contains many changes made by @ewanmellor, thanks!

modules/swagger-codegen/src/main/resources/swift3/APIHelper.mustache

@@ -0,0 +1,42 @@
+// APIHelper.swift
+//
+// Generated by swagger-codegen
+// https://github.com/swagger-api/swagger-codegen
+//
+
+import Foundation
+
+class APIHelper {
+    static func rejectNil(_ source: [String:Any?]) -> [String:Any]? {
+        var destination = [String:Any]()
+        for (key, nillableValue) in source {
+            if let value: Any = nillableValue {
+                destination[key] = value
+            }
+        }
+
+        if destination.isEmpty {
+            return nil
+        }
+        return destination
+    }
+
+    static func convertBoolToString(_ source: [String: Any]?) -> [String:Any]? {
+        guard let source = source else {
+            return nil
+        }
+        var destination = [String:Any]()
+        let theTrue = NSNumber(value: true as Bool)
+        let theFalse = NSNumber(value: false as Bool)
+        for (key, value) in source {
+            switch value {
+            case let x where x as? NSNumber === theTrue || x as? NSNumber === theFalse:
+                destination[key] = "\(value as! Bool)" as Any?
+            default:
+                destination[key] = value
+            }
+        }
+        return destination
+    }
+
+}