add parsing support for msgctxt field of poitems

The gettext format specifies a msgctxt field for translated items. An
example of how to use this field has been added to the tests and the
parsing of this attribute has been implemented.

lib/po.js

@@ -139,6 +139,10 @@ PO.parse = function (data) {
             item.msgstr[plural] = extract(line);
             context = 'msgstr';
         }
+        else if (line.match(/^msgctxt/)) { // Context
+            finish();
+            item.msgctxt = extract(line);
+        }
         else { // Probably multiline string or blank
             if (line.length > 0) {
                 if (context === 'msgstr') {
@@ -160,6 +164,7 @@ PO.parse = function (data) {
 
 PO.Item = function () {
     this.msgid = '';
+    this.msgctxt = '';
     this.references = [];
     this.msgid_plural = null;
     this.msgstr = [];