Add support for database-synced tasks with multiple Ats (resolves #94)

 * Accept comma-separated string for `at` from  task in database and
   split this into an `at` array to make use of normal Clockwork support
   for events with multiple `at`s

 * Updated ManagerWithDatabaseTasks and DatabaseEventSyncPerformer to
   handle multiple `at`s
Version 0.7.5
Add pid option so it can be used in monitoring tool

- Refactored bin/clockwork
- Moved re-usable behavior into `cli.rb`
Slight refactor to clockworkd