Added boolean-field 'active' to User-model.

Added an 'active-flag' to the User-Model-Class as boolean field which will
be stored in the database.
Removed trailing whitespaces in User-class.
Added method to query database by boolean-field.

Implemented a method within the DB-DAO to query by a booelan field.
New web-interface returning all active users.

Implemented a new web-interface which returnes all user-objects in the
database, which have the flag 'active' set to true.
API-Docs, non-functional change.
getEntitiesByBooleanField-method now uses generics

Refactored the newly created getEntitiesByBooleanField-method by using
the strategy of Java-generics. So the returned database-objects are
already casted to the passed type on the DAO level.
Also adapted the existing call of the method.
API-Docs and formatting. Non-functional change.
Merge pull request #26 from chrismayer/extend-user

Added active-flag for user-objects