Update wmacore_dev.js

Adding a listener for a getcoords-request. Now it is possible to get the coordinates of the boundaries of the WikiMiniAtlas from the iframe.

Example:
```JavaScript
$(window).bind('message', function(e) {
    console.log(e.originalEvent.data);
});

var $iframe = $('iframe');

$iframe[0].contentWindow.postMessage('{\"getcoords\":1}', location.protocol + $iframe.eq(0).attr('src'));