Popup menu implemented.
NOTE: user must click on the JTable for it to work.
Fixed the popupmenu null pointer execption
Fixed pupup delete option - it didn't have a listener applied, and the add option had the delete and add listeners applied, removed delete listener from add option
Refactored GUI, MusicPlayer and ShiBase.  Added primary key id for songs in ShiBase. Moved MusicPlayer logic that was in GUI over to MusicPlayer.
Added MusicLibrary class, Refactored GUI class, minor changes to MusicPlayer class