Making ChPayloads exported

I'm trying to do somehing like the following (adding another handler to the web-service for some other purposes):

```go
package main

import (
	"fmt"
	"github.com/elimisteve/go.github/webhooks"
	"net/http"
	"os"
)

func webListener(port string, payloads chan *webhooks.GitHubPayload) {
	webhooks.chPayloads = payloads
	http.HandleFunc("/postreceive", webhooks.WebhookHandler)
	http.HandleFunc("/anotherPath", handler)
	if err := http.ListenAndServe(":"+port, nil); err != nil {
		fmt.Printf("Error in webListener ListenAndServe: %v\n", err)
		os.Exit(1)
	}
}

func handler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[2:])
}

func main() {
	payloads := make(chan *webhooks.GitHubPayload)
	go webListener("8888", payloads)

	for {
		payload := <-payloads
		fmt.Printf("Unmarshal'd callback data:\n%+v\n\n", payload)
	}
}
```

However, I'm running into the problem that this is impossible unless chPayloads is exported. This pull request makes chPayloads exported to allow for this kind of use.