add certificate interface file

src/cert/interface.js

@@ -0,0 +1,22 @@
+'use strict';
+
+// -------------------------------------------
+//  This file just describes the interface
+//
+//  New backends can start from here
+// -------------------------------------------
+
+exports = module.exports = {
+    getCertificate: getCertificate
+};
+
+var assert = require('assert');
+
+function getCertificate(domain, options, callback) {
+	assert.strictEqual(typeof domain, 'string');
+	assert.strictEqual(typeof options, 'object');
+	assert.strictEqual(typeof callback, 'function');
+
+    return callback(new Error('Not implemented'));
+}
+