properly escape all unprintable characters

writing messages should no be in line with gettext tools. I tested
using msgcat, it provides the same results.

For some common use-cases I wrote explicit tests, for uncommon and
even unwanted use-cases I wrote one test to make sure pofile works
like msgcat for those messages

lib/po.js

@@ -236,8 +236,27 @@ PO.Item.prototype.toString = function () {
 
     // reverse what extract(string) method during PO.parse does
     var _escape = function (string) {
-        string = string.replace(/\\/g, '\\\\');
-        return string.replace(/"/g, '\\"');
+        // don't unescape \n, since string can never contain it
+        // since split('\n') is called on it
+        string = string.replace(/[\x07\b\t\v\f\r"\\]/g, function (match) {
+            switch (match) {
+                case '\x07':
+                    return '\\a';
+                case '\b':
+                    return '\\b';
+                case '\t':
+                    return '\\t';
+                case '\v':
+                    return '\\v';
+                case '\f':
+                    return '\\f';
+                case '\r':
+                    return '\\r';
+                default:
+                    return '\\' + match;
+            }
+        });
+        return string;
     };
 
     var _process = function (keyword, text, i) {
@@ -286,6 +305,7 @@ PO.Item.prototype.toString = function () {
             } else {
                 text = isArray(text) ? text.join() : text;
                 var processed = _process(keyword, text);
+                //handle \n in single-line texts (can not be handled in _escape)
                 for (var i = 1; i < processed.length - 1; i++) {
                     processed[i] = processed[i].slice(0, -1) + '\\n"';
                 }