add 4-space soft-tabs to qtconsole

pressing tab inserts four spaces, so there should never be any '\t' characters
inserted by typing.

closes gh-900
closes gh-513

IPython/frontend/qt/console/console_widget.py

@@ -1158,7 +1158,10 b' class ConsoleWidget(LoggingConfigurable, QtGui.QWidget):'
 
             elif key == QtCore.Qt.Key_Tab:
                 if not self._reading:
-                    intercepted = not self._tab_pressed()
+                    if self._tab_pressed():
+                        # real tab-key, insert four spaces
+                        cursor.insertText(' '*4)
+                    intercepted = True
 
             elif key == QtCore.Qt.Key_Left: