Create the base "TrackedEntity" class for all entities

Create a base entity superclass, called "TrackedEntity", which includes
all the logic related to the "id"s attributes, the "modified",
"created" and "deleted" lifecycle-related attributes, and the
"Timestampable" and "SoftDeleteable" Doctrine extensions.
Refactor Core Entities to inherit from TrackedEntity

Use the new "TrackedEntity" base class and move common logic to that
superclass.
Refactor Security Entities to inherit from TrackedEntity

Use the new "TrackedEntity" base class and move common logic to that
superclass.
Refactor Projector Entities to inherit from TrackedEntity

Use the new "TrackedEntity" base class and move common logic to that
superclass.
Merge pull request #35 from nxtelevision/31-create-entity-base-class

Create entity base "TrackedEntity" class (resolves #31)