don't remove \n characters from written po file

in Item.toString, all \n characters are removed from the output.
The gettext tools however leave those characters intact. This
will now produce the same output as tools like msgcat.

lib/po.js

@@ -262,6 +262,9 @@ PO.Item.prototype.toString = function () {
             } else {
                 text = isArray(text) ? text.join() : text;
                 var processed = _process(keyword, text);
+                for (var i = 1; i < processed.length - 1; i++) {
+                    processed[i] = processed[i].slice(0, -1) + '\\n"';
+                }
                 lines = lines.concat(mkObsolete + processed.join('\n' + mkObsolete));
             }
         }