Extend Snap Slider to modify image sources.

In a similar fashion to `data-snapslider-bg`, allow images with the attribute `data-snapslider-src` to have their src modified on the fly using the patterns in `data-ss-src-normal`, `data-ss-src-large` & `data-ss-src-largest`.
Merge pull request #1 from MikeTinson/master

Extend Snap Slider to modify image sources.