Added OVSDB Set Notation and the required special JSON marshaling

Signed-off-by: Madhu Venugopal <madhu@socketplane.io>
Added OVSMap and the corresponding custom marshaling

Signed-off-by: Madhu Venugopal <madhu@socketplane.io>
Merge pull request #16 from mavenugo/map_marshal

OVSSet and OVSMap notation and Custom JSON marshaling