Fix typo and incorrect type in responses.go

I'm working on a tool that exports your favourite songs from Pandora as JSON.

Your library has made this really easy but I did have to change `FeedbackResponse.IsPositive` to type `bool` to get it to work.

This is what I was doing to cause the error (simplified):

```go
package main

import "github.com/cellofellow/gopiano"

func main() {
	username := "username"
	password := "password"

	// just setting up a new client
	client, _ := gopiano.NewClient(gopiano.AndroidClient)
	client.AuthPartnerLogin()
	client.AuthUserLogin(username, password)

	// getting the stations
	stationsResult, _ := client.UserGetStationList(false)
	stations := stationsResult.Result.Stations

	// assuming station exists and has at least one feedback response
	station := stations[2]

	// panic occurs during unmarshalling of response
	_, err := client.StationGetStation(station.StationToken, true)
	if err != nil {
		panic(err)
	}
}
```

I also fixed a typo in `Station.Feedback.ThumbsDown` with the JSON tag.