Unsets the sentinel on successful check now

Also explicitly handles the UNSET sentinel state.
Nearly API compat with diesel.buffer.Buffer

 * Added clear_term() method.
 * feed() now checks the sentinel and returns if there is a match.
 * Handles (somewhat hackishly) the ANY terminal.
 * CRUCIAL: Py_None ref counting bugfix (Py_RETURN_NONE to the rescue).