add heatcanvas for 51map extension

add heatcanvas support for 51ditu.com

var heatmap = new HeatCanvas51Layer(maps, {'step':0.5, 'degree':HeatCanvas.LINEAR, 'opacity':0.7});
var data=[[116,39,20],[121.503594,31.234136,33],[118.891178,32.115234,12]];
for(var i=0,l=data.length; i<l; i++) {
    heatmap.pushData(data[i][0]*100000, data[i][1]*100000, data[i][2]);
}
maps.addOverLay(heatmap);