Add test support for CI

[FriedrichFroebel]

There currently are no automated tests on Travis CI. While I would be able to enable these for commits to the master branch, they would not work for pull requests as I would have to expose an Opencaching.de password to the public due to Travis not allowing access to secret variables on external contributions (which perfectly makes sense to avoid exposing it due to malicious contributors).

We might avoid this by mocking network calls. There would be the option to set-up an own site with the Opencaching.de server code to use for each test, but the tests would take a lot of time afterwards. Using the real site for each commit on the master branch and for pull requests might introduce additional traffic which I am not sure either.

Additionally all tests requiring write access (or existing logs) at Opencaching.de have currently been disabled. This is the case as I do not want to do development tests on the production server. We might come up with a appropriate solution for this in the same step (or in another one). (There is a public test server, but it currently is in maintenance mode and OKAPI e-mails are not sent for now.)

[FriedrichFroebel]

The last paragraph is not valid any more since 76e4ca7 as the e-mail problem with the test server has been fixed in June.