Fix a bug that failed tasks are not retried

Previously all task instances share the 'retried' variable. Now
it's moved to the namespace of the function call
Merge branch 'feature/challenge' into develop
Add a task to sync user's historical data
Add a command to manage.py to list subscriptions
Add a manage command to add subscription
Fix the subscribe command
Fix a bug that's causing Update save failure
Add default ordering for Activity, Sleep, Update
Add Sleep to API; Fix ordering of Update
Implement handlers for intraday activity API
Fix a task max_retry bug

Fix the bug that when max_retry is 0, it is overwritten by class configuration.
Get multiple days of intraday activity data
Fix wrong arguments to get_intraday
Return only url instead of actually making a request
Pull all intraday data for all resources
Add unique constraint for IntradayActivity(user, activity_time)
Work around MySQL float approximation problem.

Only store float data on some predefined places.
Merge branch 'feature/data-feed' into develop

Supported data feeds are:
* activities
* sleep
* intraday activities