Various+Issue #1244162: changed variable_set management, new hook_cronapi syntax and features, features module support

Issues (various) by gotheric, dwightaspinwall: Support for external ping service, new global semaphore handling, fixes for some PHP notices, fixed Drupal VERSION handling

Improved external cron.php to support installation path like sites/all/modules/... Thanks to voidberg #787728

MAJOR REFACTORING: Job data is now stored in a separate table, new interface to select cron rules from a configurable drop down list, performance improvements, bugfixes in aborted jobs management

Fix about running cron jobs via status page and custom jobs initialization (thanx to rsevero #647022, and mrsecret #621878), fix in weight management.

Added context default rule, added job weight (to change execution order), added time limit settings (to change max_execution_time), settings page improved, fix for cron_semaphore usare in direct elysia cron call, fix for context disable not working, code cleanup.