Hide _imgNode until image has loaded

Currently, while images are loading, they are shown as an unscaled overlay, which can make a map appear like the following temporarily:

![screen shot 2014-08-12 at 11 23 14 pm 1](https://cloud.githubusercontent.com/assets/2966080/3901807/74f86cde-22b2-11e4-8531-fafd4282d58d.png)

This is corrected once the image has loaded, but is distracting in the meantime. This change hides the imgNode while the image is loading, and then reveals it once the image has loaded to remove this effect.