Add Zone class

* Zone class provides the group of networks that will be shared the same
  captive portal environment and functions to manipulate the networks data
  which the Session and Client will be registered to the Zone and Network
  respectively.

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>
Add ZonesPool class

* ZonesPool class provides the pool of zones and the functions to
  manipulate the Zone data. It's necessary to have at least one zone for
  captive portal environment.

  ZonesPool could point to the Zone that Network relied, it useful for
  the operation that needs to interacting with Zone and Network,
  Session and Client as an example.

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>