Change signature for exists to match node spec.

Author: izaakschroeder

lib/MemoryFileSystem.js

@@ -196,7 +196,7 @@ MemoryFileSystem.prototype.normalize = normalize;
 
 // async functions
 
-["exists", "stat", "readdir", "mkdirp", "mkdir", "rmdir", "unlink", "readlink"].forEach(function(fn) {
+["stat", "readdir", "mkdirp", "mkdir", "rmdir", "unlink", "readlink"].forEach(function(fn) {
 	MemoryFileSystem.prototype[fn] = function(path, callback) {
 		try {
 			var result = this[fn + "Sync"](path);
@@ -207,6 +207,10 @@ MemoryFileSystem.prototype.normalize = normalize;
 	};
 });
 
+MemoryFileSystem.prototype.exists = function(path, callback) {
+	return callback(this.existsSync(path));
+}
+
 MemoryFileSystem.prototype.readFile = function(path, optArg, callback) {
 	if(!callback) {
 		callback = optArg;