Moves assigning event listeners into the js code.

index.html

@@ -29,14 +29,14 @@ td.visible {
   </fieldset>
 
   <section>
-  <button id="btnLeft" onclick="btnLeft_onClick()">Left</button>
-  <button id="btnUp" onclick="btnUp_onClick()">Up</button>
-  <button id="btnDown" onclick="btnDown_onClick()">Down</button>
-  <button id="btnRight" onclick="btnRight_onClick()">Right</button>
+  <button id="btnLeft">Left</button>
+  <button id="btnUp">Up</button>
+  <button id="btnDown">Down</button>
+  <button id="btnRight">Right</button>
   </section>
   <section>
-    <button id="btnStart" onclick="btnStart_onClick();">Start</button>
-    <button id="btnRedraw" onclick="btnRedraw_onClick();">Redraw</button>  
+    <button id="btnStart">Start</button>
+    <button id="btnRedraw">Redraw</button>  
   </section>
   <script src="main.js"></script>
 </body>