Integration external test cases

[christophlingg]

there are a bunch of existing test cases (from nominatim, komoot, ...) that can be integrated into our own test framework to help us finding bugs an improvments

[karussell]

This is the most important issue for now to validate that #254 is an improvement:

• integrate this repo into this repo and run py.test --photon-url 'http://<host>:<port>/api?'
• see other projects or Fuzzy Testing pelias/acceptance-tests#109

[karussell]

@hbruch Let us move the discussion from #288 to here. Furthermore to increase stability I would like to execute the geocoder-tester suite after every export and only publish if every is green. So I somehow need to extract the successful tests only. This can be done via the --compare-report option, as discussed here. Do you think that makes sense or is there a better approach?