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