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
Merge pull request #96 from robisenberg/allow_multiple_ats_for_database_tasks

Add support for database-synced tasks with multiple Ats