spawn should capture global variables

[JeffBezanson]

@spawn expressions correctly capture local variables the way closures always do, but this does not work for global variables defined at the prompt:

julia> d=drandn(4,4)
DArray((4,4),RemoteRef(1,1,2),[1,2],[1,3,5],2,1,GlobalObject())

julia> @spawn d[1]+1
RemoteRef(2,1,4)

julia> exception on 2: in anonymous: d not defined

This should be made to work somehow. Basically we need to surround the spawn with let d=d ... for each global it references.