Gutenberg: Initialize and remove timer during mounting and unmounting.

The timer is used for checking user inputs.

See #13.

block.js

@@ -488,13 +488,19 @@
 			this.onChange = this.onChange.bind( this );
 			this.onKeyDown = this.onKeyDown.bind( this );
 
-			this.timer = null;
-
 			this.state = {
 				value: props.value
 			};
 		}
 
+		componentDidMount() {
+			this.timer = null;
+		}
+
+		componentWillUnmount() {
+			clearTimeout(this.timer);
+		}
+
 		onChange( event ) {
 			const inputValue = event.target.value;