Merge pull request #1 from jweese/compact

allow extraction of full sentence rules
Merge branch 'compact' of https://github.com/jweese/thrax into compact
Moved _S symbol out of vocabulary.
Don't ignore rule symbol limits for full sentence rules

The full-sentence rules need to be as well-formed as any other.
The only difference is that they're allowed to ignore the max
span of initial phrase pairs for their left hand side.
Only use _S label for rules with long LHS spans

For very short training pairs, like dictionary entries, we don't
want to assign the (rather uninformative) label _S when something
better like a POS is available.

We reserve the special "_S" symbol for rules that could only be
extracted because the rule span limit is lifted for rules that
span the whole sentence.