Add sys.excepthook.

Update docstring and library reference section on 'sys' module.
New API PyErr_Display, just for displaying errors, called by excepthook.
Uncaught exceptions now call sys.excepthook; if that fails, we fall back
    to calling PyErr_Display directly.
Also comes with sys.__excepthook__ and sys.__displayhook__.

Include/pythonrun.h

@@ -59,6 +59,7 @@ DL_IMPORT(struct symtable *) Py_SymtableString(char *, char *, int);
 
 DL_IMPORT(void) PyErr_Print(void);
 DL_IMPORT(void) PyErr_PrintEx(int);
+DL_IMPORT(void) PyErr_Display(PyObject *, PyObject *, PyObject *);
 
 DL_IMPORT(int) Py_AtExit(void (*func)(void));