Bug 1042213 - Update Task API to include attempts by users for a particu...

Updated the task API to include TaskAttempt values.
This supports GET and POST requests.

Also modified the unit tests for the changes made to API.

This can be tested like:

GET:
>>> requests.get('http://localhost:8000/api/v1/task/2/',headers={'Authorization':'Token 1e1d43aefd834913de3fa48c7a81a8e2287c967ff'}).text

POST:
>>> data = {"name": "Sample Task", "short_description": "Task Desc", "instructions": "Task Inst", "execution_time": 30, "is_draft": 0, "project": "Mozillians", "team": "Web QA", "type": "Automated test", "repeatable": 0, "difficulty": 1, "why_this_matters": "Task matters", "keyword_set": [{"name":"Testing"},{"name":"Webqa"}], "taskattempt_set":[{"user":"existing_user@users.com", "state":0}]}

>>> jdata = json.dumps(data)

>>> requests.post('http://localhost:8000/api/v1/task/',data=jdata,headers={'Content-Type': 'application/json', 'Authorization':'Token 1167810bf3c5c80897f4a443410e51431c1189c49abc'}).text