Some contributions and ideas by Alexander Schmolck: add a keybinding

Some contributions and ideas by Alexander Schmolck: add a keybinding
to call pychecker on the current file, add a face for pseudo
keywords self, None, True, False, and Ellipsis.  Specifically,

(py-pychecker-command, py-pychecker-command-args): New variables.

(py-pseudo-keyword-face): New face variable, defaulting to a copy of
font-lock-keyword-face.

(python-font-lock-keywords): Add an entry for self, None, True, False,
Ellipsis to be rendered in py-pseudo-keyword-face.

(py-pychecker-history): New variable.

(py-mode-map): Bind C-c C-w to py-pychecker-run.

(py-pychecker-run): New command.