added StartPage_OnDrop() function

Fixes:http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4204
@Benglin Please take a look.
Let's try to make this as fail-safe as possible (even I know the file list is not likely going to be empty, but it's still a good practice):

```cs
var files = e.Data.GetData(DataFormats.FileDrop) as string[];
if (files != null && (files.Length > 0))
{
    if (dynamoViewModel.OpenCommand.CanExecute(files[0]))
    {
        dynamoViewModel.OpenCommand.Execute(files[0]);
        e.Handled = true;
    }
}    
```