Fix 2 bugs

Sorry that I did two things at once. I was going to submit these back-to-back, but why bother? Let's just get it over with.

Zeroth thing: Added an [.editorconfig](http://editorconfig.org/) file to make it easier for developers to abide by the codebase's style.

The first bug that was fixed was the one I mentioned in [a PR](https://github.com/oblador/angular-scroll/pull/96) yesterday: It was not possible to have an "animation" lasting 0 seconds. I figure it was just due to slick use of falsy values; a "duration of 0" should definitely be possible.

The second bug I fixed was one resulting from free variables. In my application, I attempted to perform 2 scroll animations simultaneously on different elements, but due to all instances of `angular.element` sharing the same `scrollAnimation` and `deferred` variables, this resulted in one element cancelling out the other's animation. So now I'm tacking the animation and deferred onto each element as instance variables, which should still allow for cancellation, but per-element.