Add formatting to index.html, handle empty strings in CE and 0 with equals

index.html

@@ -13,18 +13,22 @@
             <div class="screen">0</div>
             <button class="clear all">C</button>
             <button class="clear error">CE</button>
+
             <button class="num">7</button>
             <button class="num">8</button>
             <button class="num">9</button>
             <button class="operator">*</button>
+
             <button class="num">4</button>
             <button class="num">5</button>
             <button class="num">6</button>
             <button class="operator">/</button>
+
             <button class="num">1</button>
             <button class="num">2</button>
             <button class="num">3</button>
             <button class="operator">-</button>
+            
             <button class="num">0</button>
             <button class="num">.</button>
             <button class="equal">=</button>