Passing the pixelRatio as a second argument of the `widthInterpolator`.
`this.devicePixelRatio` was undefined because called statically.

The value is now passed to the function called within the constructor.
widthInterpolator + pixelRatio unit test.
Merge pull request #109 from BBC-News/feature-84

widthInterpolator to accept a pixelRatio parameter
cssBackground: Documentation and example.
Imager.getClosestValue tests
Rename methods.

- replaceImagesBasedOnScreenDimensions -> updateElement
- changeImageSrcToUseNewImageDimensions -> filterUrl
- removed determineAppropriateResolution (getClosestValue does the job)
ImageElementStrategy and BackgroundImageStrategy to deal with CSS backgrounds
More JSDoc please.
JSHint compliance and one-time build of availableWidths.