add Aspect Fill to Rect

Submitting the PR again as the previous one was targeted to the dev branch.

`getCenteredFill( const RectT<T> &other, bool contract )` is the counterpart of `getCenteredFit()`, a basic feature I've needed in a lot of projects.

Centered fill is the [CSS equivalent](http://www.w3schools.com/cssref/playit.asp?filename=playcss_background-size&preval=cover) of `background-size:cover`, with centered fit being `background-size:contain`.

You can test how it works with this simple app https://gist.github.com/araid/c6a59642e511cf3e0112