Retina display

@lifeiscontent pointed out in this issue https://github.com/jrief/angular-retina/issues/12 that Apple now suggests to use another infix for Retina images. See https://developer.apple.com/library/safari/documentation/NetworkingInternet/Conceptual/SafariImageDeliveryBestPractices/ServingImagestoRetinaDisplays/ServingImagestoRetinaDisplays.html for details.
In order to be backwards compatible this patch keeps the legacy ``@2x`` infix but gives the user the possibility to use the now suggested ``_2x`` infix.
This PR has been rebased to my PR #265 (Postprocessor), which is still pending.