Fix #53: Enable end-to-end testing, new test cases

[51-code]

Enables end-to-end testing:

• RelpServer provides methods for tests to start up and close the RelpServer.
• RelpServer stores all payloads, that can be retrieved by the test after the HTTP request is made. The payloads are cleared after each test.

New tests for:

• End-to-end tests for payload regex splitting
• Testing the NPE from #50
• Sending multiple requests in a row

Questions/worries:

• More test cases ?
• Should the Thread running Main be closed somehow after tests?
• Should there be testing for correctness of HTTP-headers in the payloads?
• Is end-to-end testing sufficient for RelpConvertion? I find it difficult to create unit tests for it.

[51-code]

Implemented requested changes.