* Remove ffmpeg from requirement, and it has to be installed via conda or manually
* Decouple pyaudio from Aserver and Arecorder
* Introduce backends interface: PyAudio(), Dummy()
* add device_info() helper function which prints audio device information in a tabular form
* Bugfix 23: Add a small delay to server booting to prevent issues when Aserver and Arecorder are initialized back-to-back
* Helper function `record` has been dropped due to legacy reasons. An improved version will be introduced in 0.3.2.