fixes #87 update LambdaAppConfig to add constructor and set method

Author: stevehu

src/main/java/com/networknt/aws/lambda/app/LambdaAppConfig.java

@@ -4,8 +4,14 @@ public class LambdaAppConfig {
     public static final String CONFIG_NAME = "lambda-app";
     private String lambdaAppId;
 
+    public LambdaAppConfig() {
+    }
+
     public String getLambdaAppId() {
         return lambdaAppId;
     }
 
+    public void setLambdaAppId(String lambdaAppId) {
+        this.lambdaAppId = lambdaAppId;
+    }
 }