Add option to do Penn Treebank style tokenization

tokenizer.perl and detokenizer.perl now have an option called -penn
which does Penn Treebank-like tokenization (English only).  This is
useful if your pipeline involves processing the corpus with tools
trained on PTB-tokenized text.

Unlike PTB, the tokenizer splits on slashes (e.g. "Monday/Tuesday"
becomes "Monday", "@/@", "Tuesday").  If using parse-de-berkeley.perl,
the option -split-slash re-joins tokens that are separated by slashes
for parsing then splits them afterwards.