Add async version of addLocalFile

Use open and readFile instead of existsSync and readFileSync. There
are still some sync functions left in the Utils.findFiles call,
but the impact is minimal compared to the readFileSync.

adm-zip.js

@@ -287,6 +287,83 @@ module.exports = function (/*String*/input) {
 			}
 		},
 
+		/**
+		 * Asynchronous addLocalFile
+		 * @param localPath
+		 * @param callback
+		 * @param zipPath optional path inside zip
+		 * @param filter optional RegExp or Function if files match will
+		 *               be included.
+		 */
+		addLocalFolderAsync: function (/*String*/localPath, /*Function*/callback, /*String*/zipPath, /*RegExp|Function*/filter) {
+			if (filter === undefined) {
+				filter = function () {
+					return true;
+				};
+			} else if (filter instanceof RegExp) {
+				filter = function (filter) {
+					return function (filename) {
+						return filter.test(filename);
+					}
+				}(filter);
+			}
+
+			if (zipPath) {
+				zipPath = zipPath.split("\\").join("/");
+				if (zipPath.charAt(zipPath.length - 1) !== "/") {
+					zipPath += "/";
+				}
+			} else {
+				zipPath = "";
+			}
+			// normalize the path first
+			localPath = pth.normalize(localPath);
+			localPath = localPath.split("\\").join("/"); //windows fix
+			if (localPath.charAt(localPath.length - 1) !== "/")
+				localPath += "/";
+
+			var self = this;
+			fs.open(localPath, 'r', function (err, fd) {
+				if (err && err.code === 'ENOENT') {
+					callback(undefined, Utils.Errors.FILE_NOT_FOUND.replace("%s", localPath));
+				} else if (err) {
+					callback(undefined, err);
+				} else {
+					var items = Utils.findFiles(localPath);
+					var i = -1;
+
+					var next = function () {
+						i += 1;
+						if (i < items.length) {
+							var p = items[i].split("\\").join("/").replace(new RegExp(localPath.replace(/(\(|\))/g, '\\$1'), 'i'), ""); //windows fix
+							if (filter(p)) {
+								if (p.charAt(p.length - 1) !== "/") {
+									fs.readFile(items[i], function (err, data) {
+										if (err) {
+											callback(undefined, err);
+										} else {
+											self.addFile(zipPath + p, data, '', 0);
+											next();
+										}
+									})
+								} else {
+									self.addFile(zipPath + p, Buffer.alloc(0), "", 0);
+									next();
+								}
+							} else {
+								next();
+							}
+
+						} else {
+							callback(true, undefined);
+						}
+					}
+
+					next();
+				}
+			});
+		},
+
 		/**
 		 * Allows you to create a entry (file or directory) in the zip file.
 		 * If you want to create a directory the entryName must end in / and a null buffer should be provided.