add Polygon and MultiPolygon getCenter

src/layer/vector/MultiPoly.js

@@ -47,6 +47,22 @@
 	L.MultiPolyline = createMulti(L.Polyline);
 	L.MultiPolygon = createMulti(L.Polygon);
 
+	L.MultiPolygon.prototype.getCenter = function () {
+		var bestCenter,
+			minArea = Infinity;
+
+		this.eachLayer(function(layer) {
+			var center = layer.getCenter();
+
+			if (center.area < minArea) {
+				minArea = center.area;
+				bestCenter = center;
+			}
+		});
+
+		return bestCenter;
+	};
+
 	L.multiPolyline = function (latlngs, options) {
 		return new L.MultiPolyline(latlngs, options);
 	};

src/layer/vector/Path.Popup.js

@@ -42,7 +42,7 @@ L.Path.include({
 
 		if (this._popup) {
 			// open the popup from one of the path's points if not specified
-			latlng = latlng || this._latlng ||
+			latlng = latlng || this._latlng || this.getCenter ? this.getCenter() :
 			         this._latlngs[Math.floor(this._latlngs.length / 2)];
 
 			this._openPopup({latlng: latlng});

src/layer/vector/Polygon.js

@@ -63,6 +63,26 @@ L.Polygon = L.Polyline.extend({
 		}
 	},
 
+	getCenter: function () {
+		var latlngs = this._latlngs,
+			len = latlngs.length,
+			i, j, p1, p2, f, center;
+
+		for (i = 0, j = len - 1, area = 0, lat = 0, lng = 0; i < len; j = i++) {
+			p1 = latlngs[i];
+			p2 = latlngs[j];
+			f = p1.lat * p2.lng - p2.lat * p1.lng;
+			lat += (p1.lat + p2.lat) * f;
+			lng += (p1.lng + p2.lng) * f;
+			area += f / 2;
+		}
+
+		center = area ? new L.LatLng(lat / (6 * area), lng / (6 * area)) : latlngs[0];
+		center.area = area;
+
+		return center;
+	},
+
 	_clipPoints: function () {
 		var points = this._originalPoints,
 		    newParts = [];