Conversion of the HTTP response headers

The method `getAllResponseHeaders` of the XHR object returns a string with all response headers. In order to make the HTTP API more comfortable to use, this PR includes a function which converts the header information into an object.

Before:

    Date: Wed, 11 Jun 2014 04:36:25 GMT
    Server: Apache
    ...

After:

    {
        Date: "Wed, 11 Jun 2014 04:36:25 GMT",
        Server: "Apache",
        ...
    }