Feature request: Mixing caption strings and MessageBoxButtons

Add additional overrides for the Show-methods to support a mix of caption strings and MessageBoxButtons. I.e.:

``` csharp
CustomMessageBox.ShowOKCancel(
    "Are you sure you want to eject the nuclear fuel rods?",
    "Confirm Fuel Ejection",
    MessageBoxButton.Ok,
    "Don't do it!");
```

Due to the number of possible combinations, this might get a little bit tedious...
