Show/hide buttons (first draft)

Added two public methods to the draw controller.
===================================
```javascript
drawControl.hideButton();
drawControl.showButton();
```

They have two parameters.
First param is the button type: polyline, polygon, rectangle, circle, marker, edit, etc.
Second param is the class name to use to show/hide, otherwise it uses 'leaflet-draw-hidden'.

Show or hide all buttons
--------------------------------------
```javascript
drawControl.hideButton();
drawControl.showButton();
drawControl.hideButton(undefined, 'hidden');
drawControl.showButton(undefined, 'hidden');
```

Show or hide one buttons
--------------------------------------
```javascript
drawControl.hideButton('circle');
drawControl.showButton('circle'));
drawControl.hideButton('circle'), 'hidden');
drawControl.showButton('circle'), 'hidden');
```

Show or hide more buttons
--------------------------------------
```javascript
drawControl.hideButton(['circle', 'marker']);
drawControl.showButton(['circle', 'marker']));
drawControl.hideButton(['circle', 'marker']), 'hidden');
drawControl.showButton(['circle', 'marker']), 'hidden');
```