Fixed the "all" command to include Mpg123Proxy
Added an overload for play to allow it to play from a given track number
mpg123 process is run in its own thread. Also added isPlaying method.
Fixed an off-by-one issue that caused the last track to be played twice
Added a boolean field for isPlaying rather than using the process object.

This improves the thread safety of Mpg123Proxy.
Threaded song playing so the PlayManager can automatically play the next song in a playlist
Slightly modified this test to better test Mpg123Proxy's new thread set up