Support for task workflow/dependencies

[mku11]

I'm planning on using tasktiger as a light-weight CI workflow scheduler. These are the basic changes that support this concept if you're interested.

Changes:

• New optional parameter depends for class Task that will be used to assign a list of task ids as dependencies.
• 2 new states: WAITING and COMPLETED.
• When a task is moved to ACTIVE state and dependencies have not completed it will move to the WAITING state.
• When a task is completed it will move any tasks that are dependent from WAITING to SCHEDULED state.
• NOTE: All tasks after completion will remain in COMPLETED state instead of removing from db!
• New optional parameter max_parallel_workers for TaskTiger.run_worker() and command line option -P --max-parallel-workers. This is a simple wrapper that will fork multiple workers for user convenience.
• Tests cases are updated, I had to add pytz to the requirements file to make it work with github ci.

This is based on a squash merge from my fork.
I'll also be creating a PR with the tasktiger-admin changes for supporting these changes.

[mku11]

This is the corresponding changes for tasktiger-admin: closeio/tasktiger-admin#31