Call gs directly in nltk.tree._repr_png_, no shell

shell=True causes ghostscript to stick around with a prompt on
MacOS X

    GPL Ghostscript 9.06 (2012-08-08)
    Copyright (C) 2012 Artifex Software, Inc.  All rights reserved.
    This software comes with NO WARRANTY: see the file PUBLIC for details.
    GS>

Note that the shell=True was deliberately introduced in 0cc3d8a
to avoid a console window from popping up, possibly
related to Windows? If so, this patch may undo that change, but
maybe we can have the best of both worlds by setting the
STARTUPINFO.wShowWindow flag instead.

http://stackoverflow.com/questions/3172470/actual-meaning-of-shell-true-in-subprocess
Fixed "About" dialog issue with Python 3.x

Attribute name "_top" was misspelled, which resulted in an error when trying to show a fallback ShowText.
tkMessageBox have been renamed to messagebox in Python 3.x. I have added another import to take this into account. From my understanding, the fallback ShowText is not needed now, but I left it just in case.
Testing
Add triples() to extract dependency triples
Fix bug :  Stanford tokenizer -- request to add -outputFormat inlineXML #735
Delete the temp file
Add option to allow zero-based node numbers in input (for zpar)
pylint
Fix bug  Stanford POS tagger behavior for empty input #722
updated download URL, resolves issue #812
moved examples to new nltk_teach repository (https://github.com/nltk/nltk_teach/commit/d208fb1d1321c367872f81c65eab8dcf48a52b74), resolves #753
Add the dependency evaluation to compute Unlabel Attachment Score and Label Attachment Score
Add tests for triples() and zero_based
Fix  Python 2/3 encoding issue #788
Fix bug Punctuation regular expression for  #805
cherry picking https://github.com/longdt219/nltk/commit/4fa2478cc24458147c0f92ac5c88874d0c7aafca; resolves #722
cherry picking https://github.com/longdt219/nltk/commit/592ba8788783bb195bdc06479ab90387cf52a0fe; cf #805
minor stylistic tweaks, resolves #805
add dependency evaluation module to parse package, cf #805
cherry-picking https://github.com/longdt219/nltk/commit/7ea05213f5c0a9dfc74b1de61cfab74d9764d983; resolves #788