render offline images asynchronously in obs list

[kueda]

Currently we're using https://github.com/koush/UrlImageViewHelper to load images from the web asynchronously in the observations list and it's great, very speedy, but when you scroll to an observation with a photo that was taken in the app, your scrolling halts and becomes jerky, presumably because we are loading the image data from a file and re-sampling every time we show it. I'd prefer to have the same smooth scrolling experience for offline photos as we have for online photos.

I'm not sure what the best approach here would be. Cache a smaller version of the photo for use in the obs list? Perform the resampling in a separate thread?

Also, somewhat lower priority but if you can come up with a working strategy for observations it would be great to apply it to the guides grid view for downloaded guides as well.