Add support for connections initiated by OF controllers

OpenFLow 1.3.2 specification introduces a new statement that OpenFlow
controllers can connect to OpenFlow switches. For the time being
we support only TCP connections from controller. In the future the switch
should only support secure connection as specification recommends.
(FlowForwarding/LINC-Switch#147).
Adjust ofp_client, ofp_channel to accepting established connection

OpenFLow 1.3.2 specification introduces a new statement that OpenFlow
controllers can connect to OpenFlow switches. ofp_client and ofp_channel
along with tests had to be refactored to support this new functionality
by accepting a socket with already established connection from
the controller.
(FlowForwarding/LINC-Switch#147).
Merge pull request #49 from FlowForwarding/issue147

Add support for connections initiated by OF controllers