Added JSON string basic escaping

[DirectX]

Basic escaping is mandatory to correctly handle situations like <node>some "quoted" text</node>

func Test() {
    xml := strings.NewReader(`<?xml version="1.0" encoding="UTF-8"?><hello>world { "a": "b", "c": ["d", "e"] }</hello>`)

    if jsonBuffer, err := xml2json.Convert(xml); err == nil {
        log.Print(jsonBuffer)

        var obj interface{}

        err := json.Unmarshal(jsonBuffer.Bytes(), &obj)

        if err != nil {
            panic(err)
        }

        log.Print(obj)
    }
}

[basgys]

It is definitely a good plan to escape strings, good work! :)

There are a couple of things to iron out though before merging it:

1. Exclude the following commit from the PR DirectX@7408620 (attributes have to be prefixed to avoid name clash)
2. Rename escapeJsonString to escapeJSONString (to please golint)
3. Add your example to the tests

[basgys]

@DirectX I would have love to see your example above in the tests, but I'm already happy with this PR. Thanks again :)