Started refactoring the code

- Can now zoom without selecting the zom tool
- Moved updateCursor to the tool class, deleted _updateCursor.js
- Removed as many references to canvas as possible, removed every global reference to context
- Added methods in Tool class to move the brush preview

views/pixel-editor.hbs

@@ -187,6 +187,7 @@
         </li>
     </ul>
 
+    <!-- TOOL PREVIEWS -->
     <div id="eyedropper-preview"></div>
     <div id="brush-preview"></div>