add 'initializing' class to initial load of the pull header.

[AndrewKoransky]

I didn't want the pull header showing on the initial load in my jquery mobile project. So I adjusted the code to add a class 'initializing' to the pull header on initial startup. This allows me to configure my own CSS to hide the pull header on startup in my jquery mobile project. Example:

div.scrollz-pull-header.initializing {
    display: none;
}

After the initial load is complete, I have to call hidePullHeader with animation turned off:

$('#myPage').scrollz('hidePullHeader',false); // animated = false to prevent 

Works great, at least in jquery mobile. Probably should be reviewed in other environments.

[zippy1978]

Thanks a lot !