flow_table: Fix license
Tests: Tweak a log message

This is part of Colin's 654fae3caa from the debugger branch.
Global replace of SwitchImpl -> SoftwareSwitch
nx_sw_switch: adding a return value to self.send()

We need access to the connection that were used to send, for replaying
purposes.
software switch and friends: refactored handler structure

now OFConnection invokes a single callback function
  on_message_received(connection, message)
when an OpenFlow message has been received and decoded. The
multiplexing to different handlers for different types hapens
in the switch.
sw_switch: Getting rid of set_io_worker
Making a setter for on_message_received
NotImplemented -> NotImplementedError
Major commit: canonacalize all log.* method invocations. Makes it much easier for fingerprinting (the "%s" is the fingerprint itself! No need to define a mask)
nx_software_switch: fix typo
nxsoftwareswitch: create nx vendor role reply with correct xid