clean up code, use ES6
clean up code, use ES6

caolan/async,steverobb/async,arigo-software/async,andalm/async,jnordberg/async,andalm/async,ecasilla/async,suguru03/async,caolan/async,ecasilla/async,hargasinski/async,rlugojr/async,suguru03/async,rlugojr/async,arigo-software/async,ezubarev/async,hargasinski/async,steverobb/async,jnordberg/async,ezubarev/async