RequireJS async plugin

Google Maps loads many JS files asynchronously, so listening just to the first script load isn't enough to check if it is ready to be used, another problem is that the regular gmaps script uses document.write, so we need to pass a `callback` parameter to make it not use `document.write` and wait for the callback call.
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Note that the async! plugin isn't really required for JSONP calls if the response is an Object. If the response is an Array or String you will need the async! plugin. [reference]

The default parameter used to set the callback name is callback, you can set a different name by passing it at the end of the dependency URL preceded by a exclamation mark (!), e.g.: async!!jsoncallback

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