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.

This is the corresponding changes for tasktiger: closeio/tasktiger#370

Changes:

• Support for WAITING and COMPLETED states.
• Dependencies listing under the task detail page.
• Task workflow visualization using vis-network on the frontend with a new endpoint /<queue>/<state>/<task_id>/graph/ in the backend.
• Addition of ID column (truncated task.id) in queue detail page.
• Flask option -b --bind for binding to specific network interface. To allow all interfaces use -b 0.0.0.0

The task.id column and the net binding are optional and independent so If you don't want them I can remove them.