Add JsonErrorMixin

This commit adds the JsonErrorMixin for formatting error message from a
given RequestHandler as JSON.
We should probably test that this works with both Tornado 3.x and 4.x.  This can be done by adding a new environment in tox.  Something like the following should work:
```
[testenv:tornado3]
basepython = python3
deps = {[testenv]deps}
commands =
    pip uninstall --yes tornado
    pip install -q "tornado>3.0,<4"
    {[testenv]commands}
```
Ick... looks like this project is not tox enabled.  That should be fixed in another iteration.  You could probably do something similar in the Travis CI file but then it would happen on each interpreter which might slow down our builds.  Probably want to handle this in tox or on a specific interpreter version.  I'll add an issue.