Allow DFP async slots to accept multiple ad sizes

I've added the option to define your ad unit sizes as an array of sizes to account for flex units. A situation where this would be important is if someone is trying to run both 728x90 and 970x66 sized units through the same header ad unit. (Or 300x250 and 300x600 ads through the same above the fold right rail unit.)

To define such an ad unit, you add a 'sizes' key to the url_vars array in the acmx_filter_ad_tag_ids ( $ids ) filter/array.

An example of how to define a flex space:
```php
array(
	        'tag'       => 'dfp_desktop_top',
	        'url_vars'  => array(
	            'tag'       => 'dfp_desktop_top',
	            'sizes'	=> array(
	            	array(
	            		'width'		=>	'728',
	            		'height'	=> '90'
	            	),
	            	array(
	            		'width'		=>	'970',
	            		'height'	=> '66'
	            	),
	            ),
	        ),
	        'enable_ui_mapping' => true
	    ),
```

This will create an ad slot that looks like:
`googletag.defineSlot('/XXX/728x90_flex', [[728,90],[970,66]], "div-id").addService(googletag.pubads())`

This update accounts for single-sized configurations as well and thus is backwards compatible with the current async tag ids array.