add the API "CancelSpotRequests" with code cleanup via "go fmt"

@mitchellh

I have added the API "CancelSpotRequests" also with code cleanup via "go fmt" this time.
```go
package main

import (
        "fmt"
        "github.com/mitchellh/goamz/aws"
        "github.com/mitchellh/goamz/ec2"
)

func main() {
        auth, err := aws.EnvAuth()
        if err != nil {
                fmt.Print(err)
                return
        }
        e := ec2.New(auth, aws.USEast)
        options := ec2.RequestSpotInstances{
                SpotPrice:    "0.001",
                ImageId:      "ami-ccf405a5", // Ubuntu Maverick, i386, EBS store
                InstanceType: "m1.small",
        }
        resp, err := e.RequestSpotInstances(&options)
        if err != nil {
                fmt.Print(err)
                return
        }
        spotRequest := resp.SpotRequestResults[0]
        spotId := []string{spotRequest.SpotRequestId}
        fmt.Println("New created Spot Request Id:", spotId)
        resp2, err := e.CancelSpotRequests(spotId)
        if err != nil {
                fmt.Print(err)
                return
        }
        spotResult := resp2.CancelSpotRequestResults[0]
        fmt.Println("Spot Request Id [", spotResult.SpotRequestId, "] State:", spotResult.State)
}
```