Eager API example from Readme.md won't work

[krynju]

I caught a glimpse of the new readme and noticed that the code example will not execute due to this small inconvenience

julia> c = Dagger.@spawn sum(b)+a
ERROR: MethodError: no method matching iterate(::Dagger.EagerThunk)

I noticed a lot of these when prototyping on DTables, so I usually work around them by wrapping the code in a function and using the macro on it with thunk arguments like this:

julia> c = Dagger.@spawn ((a,b)->sum(b)+a)(a,b)
EagerThunk (running)

julia> fetch(c)
13.881714780482696

I didn't really dig much into this, but I noticed this issue that might be related #216

[jpsamaroo]

Thanks for the catch! Fixed in 247f0ba.