feat(jqm-class): add jqm-class directive, like ngClass

I couldn't go with decorating ngClass like you suggested - ngClass directive will $watch attr.ngClass, and if we try to one-time set ngClass with a jqmClass trick then set a real ngClass, it will stay with the real attr.ngClass.

https://github.com/angular/angular.js/blob/master/src/ng/directive/ngClass.js#L8

So I ended up just basically copying angular's ngClass code, and tried to shorten it a little bit.