Added WASAPI exclusive host

[jesnor]

In exclusive mode only one stream can be opened per device. This is different from the ASIO host which does its own mixing of multiple streams using the sample format of the device. I don't think this is a good idea because it can lead to audio artifacts like lack of precision and overflow. So it's better to let the application do the mixing, preferably using f32 or f64 samples which are then converted to the device sample format.

Also removed duplicated code in the WASAPI Device methods for creating input/output streams.