Added some HTML code to make the digest e-mail look more nicer

It would be better to have some formatting options in the module it self, but this was quick and painless workaround.

- added header and footer in the digest e-mail on line 545

return  "<strong>Hi! this is your weekly newsletter issue. You getting this e-mail because you are member of the site yourwebpage.com</strong><br /><hr /><br />" . $output . "<br /><hr /><strong>If you do not wish to get this newsletter click on the link below and unsubscribe yourself. <br /></strong>";

-added digest content type formatting on the line 535

 $new_name = "<br><strong><u>" . check_plain(variable_get('simplenews_digest_content_rename_' . $type, node_get_types('name', $type))) . "</u></strong>";