add new method to handle HTML documents except UTF-8

This commit  adds new method to handle HTML documents other than UTF-8.

The new method is NewConvertedDocument(url string, charset string).

It takes two arguments.

First arg is  same as NewDocument method.  

Second arg is original character code of the website.

Return value is the same as NewDocument method. 

To convert character code, I use [iconv-go](https://github.com/djimenez/iconv-go).

Example is below.

```go
	// In case of HTML except utf-8, use NewConvertedDocument method
	other_doc, _ := NewConvertedDocument("http://mixi.jp/", "euc-jp")
	other_doc.Find(".loginButton").Each(func(_ int, s *Selection) {
		text, _ := s.Find("input").Attr("alt")
		fmt.Printf("output: %s \n", text)
	})
``` 

I wrote this example in goquery/example_test.go.