Add Ctrl-+/- to increase/decrease font size.

I'm not sure yet how to change the window size automatically to keep
the window at a constant row/columns size, but at least now users can
adjust the font size, which is a major usability issue.

Also, fixed minor typo in a variable name.

IPython/core/usage.py

@@ -394,7 +394,8 The keybindings themselves are:
 - ``M-d``: delete next word.
 - ``M-Backspace``: delete previous word.
 - ``C-.``: force a kernel restart (a confirmation dialog appears).
-
+- ``C-+``: increase font size.
+- ``C--``: decrease font size.
 
 The IPython pager
 =================

IPython/frontend/qt/console/console_widget.py

@@ -536,6 +536,13 class ConsoleWidget(Configurable, QtGui.QWidget):
         font.setStyleHint(QtGui.QFont.TypeWriter)
         self._set_font(font)
 
+    def change_font_size(self, delta):
+        """Change the font size by the specified amount (in points).
+        """
+        font = self.font
+        font.setPointSize(font.pointSize() + delta)
+        self._set_font(font)
+
     def select_all(self):
         """ Selects all the text in the buffer.
         """
@@ -895,7 +902,7 class ConsoleWidget(Configurable, QtGui.QWidget):
             elif key == QtCore.Qt.Key_O:
                 if self._page_control and self._page_control.isVisible():
                     self._page_control.setFocus()
-                intercept = True
+                intercepted = True
 
             elif key == QtCore.Qt.Key_Y:
                 self.paste()
@@ -904,6 +911,14 class ConsoleWidget(Configurable, QtGui.QWidget):
             elif key in (QtCore.Qt.Key_Backspace, QtCore.Qt.Key_Delete):
                 intercepted = True
 
+            elif key == QtCore.Qt.Key_Plus:
+                self.change_font_size(1)
+                intercepted = True
+
+            elif key == QtCore.Qt.Key_Minus:
+                self.change_font_size(-1)
+                intercepted = True
+
         #------ Alt modifier ---------------------------------------------------
 
         elif alt_down: