Added getStyle helper method

Added [getStyle helper method](https://github.com/stucox/Modernizr/blob/f3bccfcc631b1813d0ad4050a4edabe5073d7a79/src/getStyle.js) for abstracting the old `window.getComputedStyle` / `elem.currentStyle` switch. Updated relevant tests to use it too.

Shaves about 1kB off a minified build with everything in it. Every little helps, eh.