add bower support

It would be great to have a `bower.json` ([define a package](http://bower.io/#defining-a-package)) in the repo for bower support. It was [also registered](http://bower.io/#registering-packages) it in the [collection of repos](http://sindresorhus.com/bower-components/) with the name `jquery.lazyload`, but not by me.

This avoids to clone the whole repo as dependency in bower (which is highly [discussed here](https://github.com/bower/bower/issues/393)).