allow extraction of full sentence rules

This requires a few changes:

1) Add a constant Vocabulary symbol for "_S".
2) If full sentence rules are allowed by the configuration file,
add a phrase pair covering the entire sentence to the set of
initial phrase pairs.
3) If we are extracting a full-sentence rule, skip the rule
length limit checks.
4) In labeling, use "_S" for the LHS label for full sentence rules.