Add a constructor to Document to handle stream by himself

[aloisdg]

Something like this would be great :

public Document(string fullName)
{
    // convert fullName to stream and do stuff
}

We could use PCL Storage but it seems that they didnt implement the code needed for portable class. I tried this :

private static async Task<Stream> OpenAsync(string fullName)
{
    var file = await FileSystem.Current.GetFileFromPathAsync(fullName);
    return await file.OpenAsync(FileAccess.Read);
}

I will look into it later. If someone knows an alternative, I am listening.