showLabel and hideLabel on CircleMarkers

L.CircleMarker objects work with Leaflet.label because they inherit from L.Path. A side effect is that they do not have showLabel and hideLabel functions like L.Marker objects do. This pull request makes it possible to write

```
var circle = L.circleMarker( [ 40, -70 ], { fillColor: "#f00", radius: 8 } )
.bindLabel( "Hello World" )
.showLabel( );
```

Where before you would need to use L.Path._showLabel(), something like:
```
var circle = L.circleMarker( [ 40, -70 ], { fillColor: "#f00", radius: 8 } )
.bindLabel( "Hello World" )
._showLabel( [ 40, -70 ] );
```

There are a few lines in dist which were changed by the build. I don't know whether it's preferred for me to write over the build for consistency or submit as-is