add CORS header
add CORS header

this solves some issues with chromecast (see
https://github.com/mafintosh/peerflix/pull/113)

karissa/webtorrent,adriagalin/webtorrent,tradle/webtorrent,fisch0920/webtorrent,jakefb/webtorrent,akiva/webtorrent,jamesliu96/webtorrent,karissa/webtorrent,webtorrent/webtorrent,webtorrent/webtorrent,nicola/webtorrent,arestov/webtorrent,ericwooley/webtorrent,joshterrill/webtorrent,FabianFrancoRoldan/webtorrent,rozifus/webtorrent,Stiveknx/webtorrent,lucamaraschi/webtorrent,gillesdemey/webtorrent,whitef0x0/webtorrent,solderzzc/webtorrent,LinusU/webtorrent,MiracleBlue/webtorrent,gillesdemey/webtorrent,loon3/webtorrent,rozifus/webtorrent,jamesliu96/webtorrent,rom1504/webtorrent,ngjermundshaug/webtorrent,sideffect0/webtorrent,nicola/webtorrent,feross/webtorrent,Stiveknx/webtorrent,dan-hill/webtorrent,tradle/webtorrent,solderzzc/webtorrent,MiracleBlue/webtorrent,fisch0920/webtorrent,ngjermundshaug/webtorrent,yciabaud/webtorrent,FabianFrancoRoldan/webtorrent,joshterrill/webtorrent,lucamaraschi/webtorrent,madmarks/webtorrent,serafinfernandez/webtorrent,beni55/webtorrent,whitef0x0/webtorrent,loon3/webtorrent,sideffect0/webtorrent,LinusU/webtorrent,ericwooley/webtorrent,dan-hill/webtorrent,madmarks/webtorrent,serafinfernandez/webtorrent,adriagalin/webtorrent,yciabaud/webtorrent,akiva/webtorrent,feross/webtorrent,jakefb/webtorrent,beni55/webtorrent