Paginator as a generic class, fixed its cache and minor documentation corrections.

* Corrected package name in ActiveJdbcFilter documentation: now is org.javalite.activejdbc.web.
* Paginator as a generic class: use as `Paginator<Item> p = new Paginator<Item>(...)`, to keep track of the type of the LazyList returned by getPage().
* Corrected Paginator.getCurrentPage() documentation: it returns 0 (not -1) if current page is not set.
* Fixed cache for Paginator.getCount(): was not caching.