reactive: Get features for 0x01
reactive: Expose SDN_OpenFlow0x01
reactive: Better debug output for OF0x01 handshake
reactive: More detailed to_string for Error messages

Related to #79
reactive: More detailed to_string for PortStatusMsg

Related to #79
reactive: fix parse function for PortState
reactive: Rework repesentation of stp_state (Closes #82)

ofp_port_state carries both a flag (whether or not the link is down) as
well as an enumeration (how the port is involved in a spanning tree
algorithm). The spanning tree states are mutually exclusive, so the code
now represents the possible states as an enumeration rather than a
collection of booleans.
reactive: Fix bug in PortStatus parsing (Closes #81)

The code was shifting the Cstruct buffer and then throwing away the
shifted buffer, and then parsing PortDescription from the beginning of
the PortStatus message. The code now uses the shifted Cstruct buffer to
parse the PortDescription.
reactive: Roundtrip testing for Port-related messages