More flexible exception handling.

Add base class for our exceptions and let several subclasses extend it.
Renamed DatabaseDAO to DatabaseDao.
Merge pull request #10 from marcjansen/exceptions

More flexible exception handling.
Merge pull request #11 from marcjansen/camel-case

Renamed DatabaseDAO to DatabaseDao.
Be explicit for the join table in User model.
Merge pull request #12 from marcjansen/user-explicit-join-table

Be explicit for the join table in User model.
Annotate updated_at with @JsonIgnore
Give FilterItems a 'Statement' operator.
Merge pull request #13 from marcjansen/statement-filter

Give FilterItems a 'Statement' operator.