Add another project user
Add wrapper for large part of SDL2 audio functionality
Add audio demo
Remove noexcept specifications

std::function(std::function&&) is only noexcept in libc++, not
libstdc++, so calculated signatire of move ctor/assignment of AudioSpec
won't match specification
Provide default value for empty callback
Add AudioSpec empty constructor
Add AudioSpec copy constructor

Since callback is non-copyable, new callback is specified
Add method to check if two AudioSpecs are equal
Add Wav class
Add demo for wav loading and playing
Merge branch 'audio'
Fix move assignments

- Add self-assignment checks
- Free resources of object which is going to be replaced to avoid
  resource leaks