Add note about JSON tests maybe being wrong.

- Add note about the JSON test maybe not being correct yet.
- Add test to checks the generated names for double underscores to be sure they
  are what is expected.

* Add conformance test for zero fields in oneofs.
* Add failures to the "expected" files.

Python 2.x doesn't detect unpaired surrogates so we have to
do that manually.

We should now have no conformance failures.

The tests are run from Travis in the same way as on other
platforms. Currently some expected failures - but only expected
in that they're what we got to start with. Will try to fix them in
other pull requests.