Submit coverage data to coveralls and add a badge (fixes #15)

Run coveralls only after the py27 build because that's the only environment which generates coverage data.

Other recipes seem to explicitly call `coverage erase`, but I'm not sure if that's actually required.
* https://github.com/petrblaho/python-tuskarclient/blob/master/tox.ini

I've also seen a recipe that calls `coverage combine`
* https://github.com/Yaco-Sistemas/django-inplaceedit/blob/master/.travis.yml
...which presumably shows the combined coverage of eg python2 and python3 branches in the code.

I can add those later if they turn out ot be required.

I've added a badge for my fork of at the moment, but if someone with admin access can enable coveralls for the `twisted/newsbuilder` repo, I'll update that before merging.

An example coverage report can be found here: 
 * https://coveralls.io/r/wallrj/newsbuilder

Fixes: #15
`${TOX_ENV}` :smile:  (the `${...}` form of variable interpolation is a good habit to have because it makes the variable name unambiguous in all cases)