add map.markers_as_json method

[rochacbruno]

To fix this line https://github.com/rochacbruno/Flask-GoogleMaps/blob/master/flask_googlemaps/templates/googlemaps/gmapjs.html#L38 fails when there is unicode data in marker

[mihxen]

add a "tojson" filter like this var raw_markers = {{gmap.markers|tojson|safe}};