feat($dragImage): service for simplifying dragImage generation

Added the `$dragImage` service, which allows much simpler generation of the `dragImage` object. If the interpolated value of the `drag-image` attribute  (introduced in #23) is a string, it is now converted to an image automatically. The generated image can be completely customized by overriding `$dragImageConfig` properties.
Thats what we want I guess. But given that the `$dragImage` service is injecting a constant config. At run time based on the context we cant override this config isnt it ? The use case I am thinking of is - What if in one dragsite you want one type of dragImage look and feel and in another dragsite on the same app - you want another dragImage. Is that something that is achievable ?