improved filtering methods #12

* `Reader::setLimit` now default to `-1`;
* `Reader::setOffset` now default to `0`;
* Complete Filter API with a `Reader::clearFilter` method to remove all registered filter functions;
* Improve sorting methods to enable easier sorting the API is now similar to the filter API:
     * `Reader::addSortBy` : add a sorting function;
     * `Reader::removeSortBy`: remove a already register sorting function;
     * `Reader::hasSortBy`: check if a function is already registered as a sorting function;
     * `Reader::clearSortBy`: remove all registered function;
     * deprecate `Reader::setSortBy` now a alias of `Reader::addSortBy`;