Paging for glossary listing

This patch includes code to implement paging functionality when listing glossary terms. Particularly useful when people need a glossary on a site with limited height.
The paging is enabled in 'Administer->Site Configuration->Glossary Settings' by checking the 'Paging' checkbox and filling out the 'Pagesize' textfield, which defaults to 10.
The core paging is used for the actual paging functionality. The terms are retrieved by the glossary_get_tree function instead of the taxonomy_get_tree function. This is to prevent retrieval of all terms when the only the terms starting with x are needed for the pager. The alphabet bar is still built based on taxonomy_get_tree.