added pivotChord support

git-svn-id: http://music21.googlecode.com/svn/trunk@3350 5c16147e-a200-11de-b0cc-ab0ad15e60d0
slightly better PivotChord test.

git-svn-id: http://music21.googlecode.com/svn/trunk@3351 5c16147e-a200-11de-b0cc-ab0ad15e60d0