Add RegSetString, RegSetUint32, and RegGetUint32

Hi, I added functions for getting and setting registry values.
Could you review this pull request?
Here is a sample code using these functions.

```
package main

import (
	"fmt"
	"github.com/AllenDang/w32"
)

func main() {
	hKey := w32.RegCreateKey(w32.HKEY_LOCAL_MACHINE, `SOFTWARE\Microsoft\Internet Explorer\Setup\8.0`)
	fmt.Printf("hKey=%v\n", hKey)
	errno := w32.RegSetUint32(hKey, "DoNotAllowIE80", 1)
	fmt.Printf("errno=%d\n", errno)
	errno = w32.RegSetString(hKey, "hello", "ほげ")
	fmt.Printf("errno=%d\n", errno)

	data, errno := w32.RegGetUint32(w32.HKEY_LOCAL_MACHINE, `SOFTWARE\Microsoft\Internet Explorer\Setup\8.0`, "DoNotAllowIE80")
	fmt.Printf("data=%d, errno=%d\n", data, errno)
	w32.RegCloseKey(hKey)
}
```

Thanks!