Added optional setPagination method that gives the possibility to add pagination links to feed pages

As reported also in issue #18 it is not possible to add links with ```rel``` and ```href``` attributes using ```setChannelElement``` or ```setLink``` methods. This is why I've implemented this new ```setPagination``` method to easily add pagination links following these documentation  http://tools.ietf.org/html/rfc5005#section-3.

This methods accepts the following parameters:
- self URL: url to the current feed page, required
- next URL: url to the next page, optional
- previous URL: url to the previous page, optional
- first URL: url to the first page, optional
- last URL: url to the last page, optional