Added writing transformed activity segments to Mongo from workouts

ActivitySegments are now transformed from data coming from MapMyFitness workouts.

- The file activity.py added to provide helper functions for writing activities to Mongo, taking into consideration externalIDs (not adding same activity twice)
- api now does the transforming from a workout to an activity segment (could be cleaner)
- flattened external_id and device_type out of metadata.  Seemed to be having some issues with the embedded dictionary.
- Settings added for which service type in order to make sure we are working on the same API and service.
- Task added for retrieving a users activities on the queue.
- Hooked up the task in the views.