Merge pull request #50 from hasufell/master

Doxygen code documentation
Merge branch 'master' of github.com:nicklan/pnmixer
Merge branch 'master' of github.com:nicklan/pnmixer
Simplify run_command()

Just let g_spawn_command_line_async() do the work. This
also allows for more complex command strings wrt #38
and makes two functions obsolete.