Same ad slot will populate multiple divs

[RobertLippens]

From issue #69

var rightSlot = googletag.defineSlot('12345/slot/name', [[300, 250]], 'div1').addService(googletag.pubads());
var topSlot = googletag.defineSlot('12345/slot/name', [[728,90]], 'div2').addService(googletag.pubads());

Will now populate two different sized ads. The prebid code param in the adUnits array must be set to div1 and div2 for this to work. I did not change setGPTAsyncTargeting and continue to pass the original slot name in so that the targeting behavior does not change.

[mkendall07]

Thanks for the PR!