core-service & ml-workers integration

In order to get to MVP, task-service must integrate with core-service and ml-workers. This involves:

automatically creating task-defs from task creation requests
providing endpoints where an ml-worker can "complete", "release", and "fail" a task

When creating a task, including task_def: {attributes} in the request data will create a new task-def alongside the task. If a task-def already exists with the specified characteristics, then that task-def is used instead of creating a new one. (resolves issue https://github.com/cognoma/task-service/issues/16)
Priorities are now integers rather than strings. Using integer priorities (1: critical, 4: low priority) makes it easier for the database to sort by priority in addition to making priority comparisons less based off of arbitrary semantics like "high" or "low". (resolves issue https://github.com/cognoma/task-service/issues/15)
ml-worker can POST to /tasks/id/complete,/tasks/id/fail, /tasks/id/release to update the status of a task (resolves issues https://github.com/cognoma/task-service/issues/13, https://github.com/cognoma/task-service/issues/14)

Added test for creating a task with the name of a nonexistent task-def to ensure the task-def is created in the same request.
Refactored task queue tests to use the complete/fail/release endpoints.

cognoma / task-service