v0.13

 - Updated Makefile and README.
 - Added constraints and minimal tags to Bubblegum::Syntax.
 - Miscellaneous cleanup of Bubblegum::Wrapper POD.
 - Updated Makefile and README.
 - Miscellaneous cleanup of Bubblegum::Syntax and Bubblegum POD.
 - Added data accessor to Bubblegum::Exception.