Added cobertura maven plugin project-wide

Details
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This makes test coverage measurements possible thanks to the cobertura plugin.
All modules have this support but only webapp can measure properly right now.

See [here](https://api.travis-ci.org/jobs/8521996/log.txt?deansi=true) for an example output of the whole maven phases including the coverage report.

Coverage targets set to %85, and maven does not fail if the coverage targets
are not reached but we have to change this behaviour asap by escalating the
coverage. Coverage targets can be configured per module.

The most effective sequence to build from console (see .travis.xml)

1) Skip tests and code instrumentation, just build first
mvn install -DskipTests=true -Dcobertura.skip=true

2) Run unit-tests
mvn test

3) Instrument code, measure test coverage and run integration tests
mvn verify

4) Report test coverge results in HTML format <module>/target/site/index.html
mvn cobertura:cobertura

NB: Actually you can skip 2, since 3 already covers unit tests. But the steps
above look more logical to me.

Ticket
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closed #10