Added function to get xml format string for environment used GA api v2.

[tomothumb]

Hi,

Google stop to support api v2 last month.
I was using GA api v2 with other library, then I found this gapi.

A product which I needed to maintenance was supported only xml format and It's kind of complicated to change structure from parse xml to JSON.

Finally I added a function to get xml format string. It will not need to change a lot the current production code(api v2 user).

Sample is here

$ga = new gapi($email, $key);
$ga->requestReportData(............);
$reportxml = $ga->getResultsXML();

as note,
This function will return brief xml format. This isn't perfect copy which GA api v2 returned before.
so I'm not sure whether this is good way or not.

See this gust. This returns string like this.
https://gist.github.com/tomothumb/7548cb89fb211cb382a2

I referred to this sample file to make this function.
https://code.google.com/p/ga-api-http-samples/source/browse/trunk/src/v2/dataFeedResponse.xml

[rogervila]

We could create the same for JSON
return $ga->getResultsJSON()