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
replaced measurements with features for bulk upload
fixed track decoding
fixed NPE with missing Content-Type header
return the right track
base interfaces (without relations) for entities
removed builder pattern from interfaces
added unit for measurement values (closes #27)
fixed group creation validation
removed subscription classes