Add coveralls.io support

Coveralls.io can display the coverage of the unit tests.

To generate coverage metrics on Travis, we run the tests with the coverage
package. After a successful build the metrics are pushed to
coveralls.io.

As we supply an empty implementation for a python-3-only function, we don't want
to track the coverage of that code path, as its execution depends on the Python version.
Add coveralls.io badge

The badge displays the current coverage of the master branch.