Removed `$`
Removed the override of `window.requestAnimationFrame`

But kept it internally as `nextTick` function.
Replacing `x in y` tests by `y.hasOwnProperty(x)`
Not testing on `imgr.cache` but on the actual pool of images
Removing `Imager.cache`

- avoids recreating a DOM node each time
- added an empty `alt` attribute
- iterating on `imgr.divs`