Expose error codes to plugin user

This way the plugin user can use the error codes in the error callback
function...

function onerror(errorCode) {
switch (errorCode) {
case window.storekit.ERR_SETUP:
break;
case window.storekit.ERR_LOAD:
break;
// etc.
}
}
Respect the debug option in client code
Merge pull request #33 from maxmoore14/master

Minor tweaks