Fix wrong sample rate while playback.

* the sample rate in libao is 'per-channel', in libav it's the global
  sample rate.
* replace deprecated function calls in libav.
* disable console output.
Fix wrong include path.
Fix warning in utils/audiopia.cpp.
Add example 'playback'.
Implement TagLib::IOStream.
Modify async framework.

async::object_interface returns an integer. if '-1' is returned, kill
the associated timer.
Modify a bunch of stuff.

- Modify 'playback' & 'spotify' examples.
- Rename 'audiopia::async::service' => 'audiopia::eventloop'
  and 'audiopia::async::object_interface' => 'audiopia::event_object_interface'
- Add option to remember Spotify's login credential.
- Modify 'audiopia::spotify::client::wait' to handle multiple states.
- Modify 'audiopia' utility to support Spotify's relogin.
- Modify 'audiopia' utility to color current track in collection-view.
- Add a help message function to all aggregator initializers.
- Add support for modifiable options in aggregator initialization.
- Python aggregators use there docstrings as help message.
- Fix typo.