Integrate Code coverage for t2wml

[saggu]

Please see the documentation - https://docs.coveralls.io/

This integration is helpful in determining what percentage of code is covered by the unit tests.

Let us know if you need help.

[zmbq]

Added to t2wml and t2wml-api. Github README page shows coverage for master branch, so check this after merging to master.

[zmbq]

Coverage calculated during tests and updates the coveralls server. There is a known github issue (lemurheavy/coveralls-public#971) that prevents the badge from always being updated. I've added the suggested workaround, hopefully we will not encounter it.

In any case, if the badge isn't updated and you want to see the actual coverage, just click on the badge and you'll see the report.

[devowit]

tested working, aside from github glitches not in our control