Fixes bugs 753 and 754.

Bug #753 - Method "void QTextDocument::undo(QTextCursor*)" missing
Bug #754 - Method "void QTextDocument::redo(QTextCursor*)" missing

Also added an unit test.

tests/QtGui/qtextdocument_undoredo_test.py

@@ -0,0 +1,31 @@
+
+import unittest
+
+from PySide.QtGui import QTextDocument, QTextCursor
+
+class QTextDocumentTest(unittest.TestCase):
+
+    def testUndoRedo(self):
+        text = 'foobar'
+        doc = QTextDocument(text)
+
+        self.assertFalse(doc.isRedoAvailable())
+        self.assertTrue(doc.isUndoAvailable())
+        self.assertEqual(doc.toPlainText(), text)
+
+        cursor = QTextCursor(doc)
+        doc.undo(cursor)
+
+        self.assertTrue(doc.isRedoAvailable())
+        self.assertFalse(doc.isUndoAvailable())
+        self.assertEqual(doc.toPlainText(), '')
+
+        doc.redo(cursor)
+
+        self.assertFalse(doc.isRedoAvailable())
+        self.assertTrue(doc.isUndoAvailable())
+        self.assertEqual(doc.toPlainText(), text)
+
+if __name__ == '__main__':
+    unittest.main()
+