mongo collection names
incorrect media type
fixed NPE while adding group members
support for user-ref media type
removed descripton from group list
path constants
fixed group encoding/schema
added created/modified times to sensor
fixed identifier property name
extended track validator
fixed track creation decoding
fixed track schema
formatted schema
fixed /users/user/tracks resource
Merge branch 'dev'

* dev:
  fixed /users/user/tracks resource
  formatted schema
  fixed track schema
  fixed track creation decoding
  extended track validator
  fixed identifier property name
  added created/modified times to sensor
  fixed group encoding/schema
  path constants
  removed descripton from group list
  support for user-ref media type
  fixed NPE while adding group members
  incorrect media type
  mongo collection names