accept url strings

This adds a very small wrapper around the existing UriAdapter so that we can assign bare url strings. So small, in fact, that you may already have considered and rejected it.

It matches for anything beginning `/^https?:\/\//`. 

```thing.image = "http://example.com/image.png"```

It seems to be a common requirement, and with strong parameters it has become more difficult to URI() a parameter in the controller. For instance, my immediate purpose is to cross-load images from dropbox by way of their javascript chooser, output from which is always a temporary url.