Fix the async calls.

This is a breaking change. But that's okay, the previous behavior for
async was just plain wrong and would crash the entire node process,
which is unacceptable. A major version bump will be needed to release
this.

lib/po.js

@@ -12,15 +12,7 @@ var PO = function () {
 };
 
 PO.prototype.save = function (filename, callback) {
-    fs.writeFile(filename, this.toString(), function (err) {
+    fs.writeFile(filename, this.toString(), callback);
-        if (err) {
-            throw err;
-        }
-
-        if (callback) {
-            callback();
-        }
-    });
 };
 
 PO.prototype.toString = function () {
@@ -54,10 +46,10 @@ PO.prototype.toString = function () {
 PO.load = function (filename, callback) {
     fs.readFile(filename, 'utf-8', function (err, data) {
         if (err) {
-            throw err;
+            return callback(err);
         }
         var po = PO.parse(data);
-        callback(po);
+        callback(null, po);
     });
 };