Add FileSetBuilder

It allows to define file sets using CE syntax instead of operators, like this:
```fsharp
let fs = [ "*Tests22.dll" ]

Target "Build" {
    files { 
        addMany fs
        add "**\bin\**\*Tests.dll"
        add "**\bin2\**\*Tests.dll"
        add "**\binaries\**\*"
        add "**\binaries2\**\*"
        exclude "**\bin2\Special*\*"
        excludeMany [ "*t2"; "*t3" ]
    }
    |> MSBuildDebug @".\out1" "Build"
    |> Log "AppBuild-Output: " 
}
```