bitpar wrapper & shortest derivation parsing

- PCFG grammars can now be parsed by bitpar;
  a bitpar stage can be used to prune the next stage
  with k-best coarse-to-fine;
  bitpar stage supports Double-DOP parsing.
- fixed bugs in shortest derivation & SL-DOP parsing,
  can now be used when grammar is read from a file.
- fragments making up the best derivations are now extracted during
  marginalization