Adding factory helper method to migrator.
Adding factory helper method to migrator.

new-xiaji/peewee,coreos/peewee,new-xiaji/peewee,zhang625272514/peewee,jarrahwu/peewee,jarrahwu/peewee,mackjoner/peewee,Dipsomaniac/peewee,bopo/peewee,stas/peewee,teserak/peewee,bopo/peewee,softside/peewee,Sunzhifeng/peewee,coreos/peewee,wenxer/peewee,coleifer/peewee,py4a/peewee,coreos/peewee,lez/peewee,Dipsomaniac/peewee,Sunzhifeng/peewee,Sunzhifeng/peewee,zhang625272514/peewee,ghukill/peewee,jnovinger/peewee,fuzeman/peewee,d1hotpep/peewee,zhang625272514/peewee,d1hotpep/peewee,jnovinger/peewee,coleifer/peewee,new-xiaji/peewee,coleifer/peewee,fuzeman/peewee,wenxer/peewee,Dipsomaniac/peewee,jarrahwu/peewee,bopo/peewee,ronyb29/peewee