Add Dozer Java Bean Mapper to pom.xml

* The dozer mapping library is used for mapping DTOs
to their respective POJOs. The mapping is now much cleaner

* Defined the dozer mapping bean in the spring configuration file. It
has been defined as a singleton to avoid reload & reinitialization of the
mapping files for each individual mapping.

* Removed old DTO mapping classes
Wiring for the buckets controller

* Implemented the endpoints to manage bucket metadata
(CRUD operations on bucket objects) and bucket collaborators.

* Added a DTO mapping class for BucketCollaborator

* Added "active" column to buckets table and a "date_added" column
to the buckets_collaborators table

* Integration tests for the buckets controller and additional test
data.
Single DTO for collaborator objects

* As per the API spec, the collaborator objects
have the same fields so it's more prudent to use a
single DTO class to represent both the RiverCollaborator
and BucketCollaborator entities.
Refactor rivers controller, service and DAO

* Removed @ResponseBody from RiversController#addFollower
The endpoint mapped to this method simply returns the HTTP
status code

* Modified the RiverService methods for managing
followers and collaborators to user AccountDTO
and CollaboratorDTO

* Refactored RiverDao#addCollaborator so that the
RiverCollaborator is instantiated and its properties set
within the method.

* Added a dateAdded property to the RiverCollaborator
entity

* Updated the controller integration tests to reflect the changes
to the endpoints and request body