In cases where a srcset attribute is specified in the markup on a fallback image, we need to remove the attribute so that src can be set by the script. this change adds a picturefill namespace for use throughout the js, and uses that namespace as a property on the image element for caching srcset values before deleting the attribute. the cached value is used for fallback consideration if no other source candidates are available. All that said, we will recommend against using srcset on the fallback image when polyfilling, as it will likely trigger a double request in browsers that support srcset but not picture (which is a temporary problem, admittedly). Fixes #167