Refactored code to abstract setting of img elements in the DOM into a single function for both picture and standard modes.
Added placeholder setting for CSS backgrounds option
Added code to set CSS BG as per breakpoint. Add temp CSS rules.
Enhance CSS Background mode by grabbing the image's src dimensions on load and set the element's height to be equal.
Abstracted setting of img element (as opposed to CSS bg) dimensions to utility functions as well.
Minor update to docs.
Return from the plugin function to allow chaining
Merge pull request #1 from getdave/feature/add-css-background-support

Feature/add css background support