Add basic Pagination mode.

This PR tries to solve problem of pagination for heavy databases with huge amount of data rows when you can't just use `SELECT COUNT(*) FROM table` to get number of rows for current version of Paginator.

Right now this proposition is only for Paginator with plans to add extra method to Eloquent (or a rewrite for `paginate()`).

Details on PR in the following picture.

![paginate](https://f.cloud.github.com/assets/578455/1696445/9a86856a-5ec4-11e3-89cc-5f43a9bdb63e.jpg)