Allow to define multiple sizes for a slot

According to the documentation, multiple sizes can be used for the same slot by using a multidimensional array, e.g.:

    googletag.defineSlot("/1234/travel/asia/food", [[468, 60], [728, 90], [300, 250]], "div-gpt-ad-123456789-1");

This change adds support for that in the gem. Previously, trying to send a multidimensional array like the above would end up generating the following Javascript array:

    [468, 60, 728, 90, 300, 250]

It is mentioned here http://support.google.com/dfp_premium/bin/answer.py?hl=en&answer=1697712 on "Define ad sizes in Google Publisher Tags"
And there's an example here http://support.google.com/dfp_premium/bin/answer.py?hl=en&answer=1638622 under "Example of an asynchronous Google Publisher Tag"