add main field
add main field

to allow publishing to npm

scottjehl/picturefill,demateu/picturefill,nucliweb/picturefill,yubin-huang/picturefill,footiealf/picturefill,rockmandew/picturefill,samiadelgado/picturefill,guardian/picturefill,mcanthony/picturefill,purposeindustries/picturefill,rockmandew/picturefill,guardian/picturefill,samiadelgado/picturefill,scottjehl/picturefill,chrismcandrew/picturefill,demateu/picturefill,iancrowther/picturefill,patrickkettner/picturefill,footiealf/picturefill,albell/picturefill,iancrowther/picturefill,chrismcandrew/picturefill,yufi113/picturefill,albell/picturefill,ddeveloperr/picturefill,purposeindustries/picturefill,ddeveloperr/picturefill,yufi113/picturefill,mcanthony/picturefill,yubin-huang/picturefill,patrickkettner/picturefill