Improve coveralls in travis build

[henrjk]

Add coveralls badge and improve build script.

Update original comment!

The introduction of the coveralls build somewhat unintentionally had all tests running at once. This used to fail which was the reason that the coverage.sh script was introduced so that the coverage would run separately for unit and integration tests and the results were combined.

However it should really be possible to run all tests at once, so that it makes sense to do so in the build. This should allow to see regressions in this respect.

The coverage script itself is kept as it can still be useful during development if only to see how one can run the commands in a shell without installing istanbul or mocha globally.

Finally the travis script now sends coverage data in a second step to coveralls with the intent to easier distinguish cases where the tests fail and where coveralls fails.

[henrjk]

The build bage an only be seen in master.

This should be good to be pulled now.

[christiansmith]

Thanks!