v2.1.0

This release solves the problem where re-initializing retina.js would cause it to search for incorrect urls for previously processed images. It solves the problem in two ways: 1) It will no longer attempt to re-process previously processed images. 2) It gives you the option of passing in an Array, NodeList, or jQuery selection of images to the retinajs function, which will cause it to attempt to process only the elements in the collection.