added explicit export list to Signal.elm
added explicit export list to Signal.elm

eeue56/core,Pisys/core,alexiamcdonald/elm-core,simonh1000/core,rick68/core,rgrempel/core,laszlopandy/core,manleyjster/core,martinos/core,vilterp/core,isaacsanders/core,Pisys/core,bradurani/core,bradurani/core,martinos/core,vilterp/core,simonh1000/core,krisajenkins/core,rick68/core,krisajenkins/core,Dandandan/core,colinmccabe/core,suryagaddipati/core,manleyjster/core,ThomasWeiser/core,rgrempel/core,Dandandan/core,isaacsanders/core,suryagaddipati/core,ThomasWeiser/core,laszlopandy/core,eeue56/core,colinmccabe/core,alexiamcdonald/elm-core