* Add commands history_get_clients, ...
* Change type of length from u16 to u32.

* Add history commands, not implemented.
* Add types for history commands.
* Fix i8 instead of u8 for rate, pitch and volume.
* Use u16 for length in commands.

* When compiled without mio, QueuedClient returns the raw fd.
* When compiled with mio, there is a register method.

* Generic methods can be used in the synchronous API. It gives the choice to use
  either specific calls or the generic ones.
* AsyncClient consists in a simple queue of requests.
* Getters only return the generic Response::Get. Callers must convert if it's an
  integer.

* Simplify fifo tests with tempfile.
* Test getter in async fifo tests.