subscribe to user data if FITAPP_SUBSCRIBE is True
make unit_type nullable, specify unique_together for timeseriesdata
change resource_type to a choice field
initial TimeSeriesDataType data
add subscriber endpoint to recieve near realtime updates from fitbit
Revert "change resource_type to a choice field"

This reverts commit 9b2518d4411d3df7062998f74e595a66b301d8b2.
add unique_together for TimeSeriesDataType
make UNIT_TYPE_CHOICES part of the Unit model
generalize make_response
use foreignkey resource types, deal with existing and new data separately
change other to METRIC, to better match fitbit