Load 2x images from any domain

We can fetch images using an `Image` object as suggested by @nebstrebor in #40. There's no need to check whether the image is hosted on another domain.

Please read commit-by-commit for details.

Should resolve:
- [40: Does not load images hosted on Amazon S3, Cloudfront and other CDNs](https://github.com/imulus/retinajs/issues/40)
- [72: Doesnt work with images uploaded in sub-domain?](https://github.com/imulus/retinajs/issues/72)
- [76: Fixes problem with checking if image path was already cofirmed](https://github.com/imulus/retinajs/pull/76)