removed TODO
activity interfaces
splitted MongoBaseEntity
mongo activity implementations
use interface in constructor
bind activity implementation to guice modules
encoders and providers
activity/activities schema
guice binding of schemas/providers/encoders
saving of activity information
Merge branch 'dev' into activities

* dev:
  corrected bulk upload
  renamed variable
  removal of track references
return the right track
base interfaces (without relations) for entities
removed builder pattern from interfaces
removed subscription classes
refactored entities
Merge branch 'dev' into activities

* dev:
  Fixed unintended recursion (closes #45)
  fixed group creation validation
  added unit for measurement values (closes #27)
  fixed NPE with missing Content-Type header
  fixed track decoding
  replaced measurements with features for bulk upload

Conflicts:
	core/src/main/java/io/car/server/core/Service.java
	rest/src/main/java/io/car/server/rest/encoding/MeasurementEncoder.java
added MongoDB dependency to DAOs
do not receive members if querying groups
implemented getMembers