Add global api version to swagger response

The property for setting the global API version already exists, however it is never set and cannot be modified by either of the Swagger events. Use the existing `AppHost.ApiVersion` value to specify this. If set, this value will appear on the bottom of the page next to the Base URL as seen on the [demo page](http://petstore.swagger.wordnik.com/).

Note: The `ApiVersion` is already specified for each [individual route](https://github.com/ServiceStack/ServiceStack/blob/master/src/ServiceStack.Api.Swagger/SwaggerApiService.cs#L182), however that does not appear anywhere on the resulting swagger page.