A bug fix and some efficiency upgrade

[Xesenix]

I found a bug when running multiple instances of appear for example something like this

$('.selector1').appear({force_process: true})
.on('appear', function() {
    console.log('hi 1');
})
.on('disappear', function() {
    console.log('bye 1');
});

$('.selector2').appear({force_process: true})
.on('appear', function() {
    console.log('hi 2');
})
.on('disappear', function() {
    console.log('bye 2');
});

when scrolled and both selector where visible disappear was triggered for both selectors.

Second fix is for disabling running appear event for elements for which it was already fired.