added retire.js to build process

-allows security vulnerability check in used npm-dependencies and own scripts
-execute with ```grunt retire```

Gruntfile.js

@@ -134,7 +134,14 @@ module.exports = function(grunt) {
 			markdown: {
 				files: [ './*.md' ]
 			}
-		}
+		},
+
+		retire: {
+            js: ['js/reveal.js', 'lib/js/*.js', 'plugin/**/*.js'],
+            node: ['.'],
+            options: {
+            }
+        }
 
 	});
 
@@ -148,6 +155,7 @@ module.exports = function(grunt) {
 	grunt.loadNpmTasks( 'grunt-contrib-connect' );
 	grunt.loadNpmTasks( 'grunt-autoprefixer' );
 	grunt.loadNpmTasks( 'grunt-zip' );
+	grunt.loadNpmTasks('grunt-retire');
 
 	// Default task
 	grunt.registerTask( 'default', [ 'css', 'js' ] );