Add test jack client.
Implement I/Q signal correction.

* Add class `IQGainPhaseCorrection`, which performs gain and phase
  correction on an I/Q signal.
* Change the `RingBuffer<T>` template to suit the needs of the
  `IQGainPhaseCorrection` class.
* Update `tests/RingBufferTest.h` test case.
* Modify `FFTBackend` to use an instance of the `IQGainPhaseCorrection`
  to preprocess incoming signal.
* Add `test` target to `Makefile`, which compiles and runs the tests.
* Update `CHANGELOG.md`.