Set Control-L to clear the screen when readline supports it.

Closes https://bugs.launchpad.net/ipython/+bug/435519

Note that before we had C-L to list completions (which tab does anyway), so
this is a minor change in behavior.  Users can get the previous behavior if
they want with a simple config setting, but C-L is the standard 'screen
clear' keybinding.

IPython/core/iplib.py

@@ -245,7 +245,7 b' class InteractiveShell(Component, Magic):'
     readline_remove_delims = Str('-/~', config=True)
     readline_parse_and_bind = List([
             'tab: complete',
-            '"\C-l": possible-completions',
+            '"\C-l": clear-screen',
             'set show-all-if-ambiguous on',
             '"\C-o": tab-insert',
             '"\M-i": "    "',