Update documentation and external facing signatures to the new interface
where Logger does not serialize to JSON.
Update destination tests to the new division of responsibilities.
Update Logger to no longer do JSON serialization.
Fix some line length issues.
Do JSON serialization in ThreadedLogWriter, since Logger no longer does
it.
News entry.
Update documentation to note API changes.
Merge remote-tracking branch 'refs/remotes/origin/master' into serialization-in-destinations-44

Conflicts:
	eliot/tests/test_output.py
Merge branch 'no-failure-fields-14' into serialization-in-destinations-44

Conflicts:
	docs/source/news.rst
Merge remote-tracking branch 'refs/remotes/origin/master' into serialization-in-destinations-44
Better Python 3 docstring.
Get rid of tai64n timestamps, and knowledge of timestamp format in eliot.filter.
Merge remote-tracking branch 'refs/remotes/origin/serialization-in-destinations-44' into no-tai64n-51
An example of outputting to stdout (useful for testing eliot.filter).