By Denny Dyroff, Entertainment Editor, The Times Nellie McKay When Nellie McKay performs on stage, her warm voice and engaging delivery seem to make beautiful musical flowers appear in the air. When people pay a visit to Longwood Gardens (Route 1, Kennett Square, 610-388-1000, www.longwoodgardens.org), there are beautiful flowers in the air and everywhere.    Send article as...