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
removed set and dict comprehensions for python 2.6 compatibility; removed u string prefixes for python 3 compatibility
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