Validate image URLs with custom validator

Previously: image model only validates for presence of URL
problem: garbage links add broken images to posts, and even blank URLs show up on the page
solution: 
1. create an ImageUrlValidator that tests for properly formatted URLs that point to the right kind of resource (.png, .jp(e)g, .gif)
2. force reload of images from database so that non-persisted urls (the ones that don't pass validation) don't show up as broken image links
Hmmm, this works:

```
$ irb
> require "uri"
> regexp = URI.regexp(%w(http https))
> s = 'http://3.bp.blogspot.com/-LrHbmtQEaOE/UrE2OqR6E9I/AAAAAAAAWhc/XMMlewnRscc/s1600/They-Told-me-I-Could-Be-Anything-.jpg'
> s =~ regexp
0
```