Convert extesea_engines_add() to hook_extesea_engine_info()

extesea_engines_add() has been removed. Instead, use hook_extesea_engine_info() to declare predefined search engines in your modules. API documentation has been added to extesea.api.php. As a result of this change, /engines/extesea.default.inc is now /includes/extesea.engines.inc and /extesea.admin.inc is /includes/extesea.admin.inc.