Rename the socket state from az_socket_not_connected toaz_socket_disconnected

This is done to be more consistent with the socket.io specs
Rename the AZSocketIOState enum item to be more consistent with the cocoa code convetiont

In the cocoa touch framework usually the enum items name is prefixed
with the type name.
Add the disconnecting socket state

According the socket.io specs the the possible states for the socket
are `disconnected`, `disconnecting`, `connecting`, `connected`.
In this case when the `[AZSocketIO disconnect]` is called the socket
goes in the state AZSocketIOStateDisconnecting while asking to the
transport to effectively close the connection, after that the socket
will be in the AZSocketIOStateDisconnected state.
Merge pull request #8 from lukabernardi/feature/socket.io_spec_state

Conform the AZSocket.io state to the socket.io spec