.external.postag: tweaks
.external.postag: allow empty and arbitrary tokens

Arbitrary tokens: supply a function txtfn to return the text
associated with a token
Ability to align Penn Treebank with RST_DT corpus

Hopefully some of the PTB bits will transfer to the PDTB as well.
Update API docs to reflect new PTB stuff
Remove obsolete text_span second arg
RST_DT: some PTB related features

Penn Treebank properties for the EDU
------------------------------------
[D] ptb_word_first     first PTB word in the EDU
[D] ptb_word_last      last PTB word in the EDU
[D] ptb_pos_tag_first  POS tag for first PTB token in the EDU
[D] ptb_pos_tag_last   POS tag for last PTB token in the EDU
[D] ptb_word_first2    first two PTB words in the EDU
[D] ptb_word_last2     last PTB words in the EDU
[D] ptb_pos_tag_first2 POS tag for first two PTB tokens in the EDU
[D] ptb_pos_tag_last2  POS tag for last two PTB tokens in the EDU

Also:

[D] same_ptb_sentence  if in the same sentence (ptb segmentation)