Add complete testsuite

[jankapunkt]

This PR completes the testsuite incl. documentation and CI.

It includes:

• test proxy project with complete toolchain
  • lint / lint-fix
  • test once / test watch / test coverage
• contribution guideline with detailed guide on how to setup and use the test proxy
• ignore config to keep the repo footprint minimal and ignore test files when publishing the package
• CI config files to run the linter and tests on any pull request

Additionally there were some fixes necessary in order to make the test suite work again:

• fixes all js files with standard linter (except examples)
• fix tests (especially client tests) in order to make coverage work
• use nyc instead of deprecated instanbul

Additional Note:

• there are still tests missing for migration 2 <--> 3 (forward/backward)
• coverage reveals some uncovered blocks, should be improved soon