Fix mass assignment error when using Imagine::Engine

I got protected attribute errors when I attempted to use Imagine out of the box, so I made these fixes on my own branch. Figured you might want them on master. This shouldn't be necessary in like a month, but at least for now people can actually use the gem as installed.

You also might want to release a new version of the gem, as the one I downloaded didn't have the `rails generators` installed. I had to load from master.