added jsontags.py as a helper to serialize objects in json
converted sequential.py from yaml tags to json tags, cf https://github.com/nltk/nltk/issues/540
removed pyyaml install instructions from install.rst as it's no longer required by nltk
removed the yaml requirement from pip-req.txt as it's no longer required by nltk
added unit tests for nltk.jsontags
Merge pull request #557 from jskda/pull_request.json_tags

Pull request.json tags, cf https://github.com/nltk/nltk/issues/540
fixed stanford tagger tuple bug in python 3
allow multiple class paths for java in nltk.internals, fixed some small bugs
decode stderr in the error message for nltk.internals.java
added interface to stanford parser, c.f. https://github.com/nltk/nltk/issues/522
added batch_parse()
implementation for tagged_batch_parse()
delete file after using NamedTemporaryFile(), added comments about windows' incompatibility without using delete=False
Merge pull request #560 from jskda/pull_request.stanford_parser

Interface to stanford parser, cf https://github.com/nltk/nltk/issues/522