Support for marker (no columns) entity

[hsn10]

marker style entities are not persisted into cassandra (didn't tested other databases) because cassandra can not store rows with 0 columns. These entities are used for storing flags. If entity exists in database, then flag is set.

Testcase at https://github.com/hsn10/ormtest/tree/nocolumns

[easility]

Since you have already mentioned that cassandra can not store rows with 0 columns, I am slightly confused that what need to be done in this case? shall we throw an exception?

[hsn10]

insert column with some special name like "_null" otherwise marker class could not be persisted.

[hsn10]

can you test if this marker class work in hbase and in memorydb?

[easility]

I just checked..It passed with MongoDb but failed with Hbase..