Setup proper coverage reporting

[roll]

Overview

For now it doesn't work well.

Tasks

• [ ] remove test itself coverage from frontend
• [ ] send to coveralls/codecov both backend/frontend coverage if repo will not be split

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Extracted from #48 for backlog.

[amercader]

If you want to omit the tests you just need a .coveragerc file with:

[run]
omit =
    goodtablesio/tests/*

Actually covering tests allowed me to find badly written tests that not weren't run as expected, but I don't mind removing them from coverage.

[roll]

@amercader I don't have pref (include/exclude). I was talking about frontend tests I don't know how to exclude for now - so happy to let it as is (if we have the same approach for backend).

So this issue could be just about proper reporting to coveralls