Add style options for marker cluster

[iiirxs]

Currently it's impossible to define in config.yml some styles for a marker cluster. The getConfigTreeBuilder() inside Ivory\GoogleMapBundle\DependencyInjection\Configuration.php expects only scalar values for marker_cluster options:

->arrayNode('options')
->useAttributeAsKey('options')
->prototype('scalar')->end()
->end()

In order to enable this functionality of defining styles for marker clusters we have to change the existent code to this one:

->arrayNode('options')
->children()
->scalarNode('gridSize')->end()
->scalarNode('maxZoom')->end()
->scalarNode('zoomOnClick')->end()
->scalarNode('averageCenter')->end()
->scalarNode('minimumClusterSize')->end()
->arrayNode('styles')
->prototype('array')
->children()
->scalarNode('url')->end()
->scalarNode('height')->end()
->scalarNode('width')->end()
->scalarNode('anchor')->end()
->scalarNode('textColor')->end()
->scalarNode('textSize')->end()
->scalarNode('backgroundPosition')->end()
->scalarNode('iconAnchor')->end()
->end()
->end()
->end()
->end()
->end()

The above code provides all options currently available by Google Maps Api