Fix problem when enum value is null

Correctly sets the member of type enum to null if needed. Avoiding an
exception because Enum.name() is null.
Adding JOhm.exists()

`JOhm.exists(Foo.class, 1L)` is like testing `JOhm.get(Foo.class, 1L)
== null` except that it directly test for existence avoiding the
complete loading of the entity and its related.
get and find can ignore properties

`get` and `find` now accepts a `String… ignoring` in order to avoid
loading certain properties when not needed.