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.
Address review comment: Fix docstring.
Merge pull request #46 from hybridcluster/serialization-in-destinations-44

Serialization in destinations, not Logger.

Fixes #44.