Issue #338: Add “priority” property to task, to take into account in scheduler.get_work

- implement handling of priority property in Task and CentralPlannerScheduler
- add documentation about task priorities
- add tests for handling priorities in CentralPlannerScheduler
Merge pull request #338 from soxofaan/feature-task-priority

Add “priority” property to task, to take into account in scheduler.get_work
Add support for email subject prefix
Fixed issue where the server was upgraded and the clients not
Support COPY from JSON format in redshift
Merge pull request #352 from fuziontech/master

Support COPY from JSON format on Redshift
record the time when the task picked up by worker
add task_search api to search for task by task_id
Merge pull request #354 from Houzz/task_search

Add task_search api to search for task by task_id
Merge pull request #353 from Houzz/time_running

Show the timestamp when tasks are picked up by workers
Merge pull request #351 from rantav/email-prefix

Add support for email subject prefix