Move Validation to REST layer

Remove Representation.to method. Previously used to fetch the underlying
Domain object to perform validation. Since validation is now done on the
Representation itself this is no longer needed.

Add @StartBeforeEnd validation annotation. Verifies that One date is
before the other.

Add NotNullValidationTheory. A base test to setup tests for a Representation
@NotNull constraints. See JavaDoc for doc.

Updated Conference, Session, Attachment, User REST layers.

Issue: #28