Refactoring for better package organization and other changes.

The main interface is renamed to OvsDBClient to represent the library's
intention of being used against any OVS capable device. Fixed methods
to be all asynchronous.

Change-Id: I9d6e0c8cc53e0bcf188d572c655c7a28e3e0dae2
Signed-off-by: Ashwin Raveendran <ashw7n@gmail.com>