Added ability to set the zoom level on a field-by-field basis, using the existing $options argument.

This adds the ability to use the $options argument to GoogleMapField::__construct() to specify the zoom level of the map when it's first created.

For example:

new GoogleMapField(new Story(array('Lat' => '-41.289996', 'Lng' => '174.784498')), 'Your Location', array('zoom' => 14)).

This will show a zoomed-in view of Wellington, NZ, rather than showing the lower half of the North Island (at the default zoom level of 8).