Expand .gitignore to support Mac and IntelliJ (credit: mipastgt). Resolves #22

Author: IntegralPilot

.gitignore

@@ -28,4 +28,10 @@ jvm-unknown-unknown.json
 **/vendor/*
 
 # Generated by Javac in the integration tests
-**/*.class
+**/*.class
+
+# Mac
+**/*.DS_Store
+
+# IntelliJ
+.idea