embed sensor in MongoMeasurement
fixed statistics
do not encode the complete track/measurement in activities
fixed deletion of entities
gravatars
fixed NPE for deleted groups in activities
allow password changes
Merge branch 'dev'

* fixes #50
* fixes #49
* closes #48

* dev:
  allow password changes
  fixed NPE for deleted groups in activities
  gravatars
  fixed deletion of entities
  do not encode the complete track/measurement in activities
  fixed statistics
  embed sensor in MongoMeasurement