Improve pagination's Bootstrap compatibility

Improved Bootstrap compatibility. With this commit, functionality for pagination alignment and size are added. You can now set size with 2 parameters.

Example:
<?php echo $users->links('centered', 'small'); ?>

This will translate to the wrapper with some extra classes:
<div class="pagination pagination-centered pagination-small">

Because of this, Bootstrap out of the box is much improved for the Pagination package. You can use all alignment and size classes for paginators from Twitter Bootstrap (as of v2.2.2).

Signed-off-by: Dries Vints <dries.vints@gmail.com>