Merge pull request #18 from mausch/net35

+ NET 3.5 build
Reintroduce Seq's overloads to .NET 3.5 version.

I think they were excluded because when Task overloads were removed type inference defaulted to Seq.
Id<'t> to the rescue !
Add explicit filter for option.
Default filter from foldr/monoid/pure didn't work in this case