add credential interface

src/main/java/CredentialInterface.java

@@ -0,0 +1,11 @@
+public interface CredentialInterface {
+
+    public void show();
+    public void edit();
+    public void delete();
+    public void getCredentials();
+    public void setCredentials();
+    public void getAsJSON();
+    public void loadCredentialsFromJSON();
+
+}