Improves extensibility; 1 internal function name change

[deprecated - use 0.4.1 - images can fail to load]

• Make 'data.element' available to slimmage.adjustImageParameters, so plugins have more information.
• Add window.slimmage.beforeAdjustSrc callback function - this allows users to inject behavior between noscript parsing and img.src edits. (say, to offer alternate criteria, like a css class, for involving images).
• Replace adjustImageSrcWithWidth with getImageInfo, make adjustImageSrc responsible for more. Simplifies certain kinds of extension, such as background image support, as the size calculation can be reused.