Fix wp-e-commerce.js to allow cart to load when caching is enabled

Addresses ".load" not firing issue with get_art ajax functionality (as discussed with @JustinSainton).

Also replaced '.load' with '.ready' for related events

Noticed a couple spots in code where there might be a need for another change, but I didn't understand what the code was trying to do so I left it alone.  See the issue #534  comments.