Add a constructor to Document to handle stream by himself

Something like this would be great :

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

We could use [PCL Storage](https://github.com/dsplaisted/PCLStorage) but it seems that they didnt implement the code needed for portable class. I tried this :

```
private static async Task 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.