Align KATCPSensor listener callback API with katcp.Sensor

also:

Improve KATCPSensor listener callback mechanism to properly deal with methods
KATResourceClient implementation WIP
WIP
WIP
Passing smoke-test WIP
Fix KATCPSensor.set_sensor_strategy, other fixes, more smoke-test progress
Working basic sensor functionality
More or less working requests
Add tests and minor fixes to inspecting-client

- Add inform-hook test, some other inspecting client test cleanups
- Correct inform handler: 'interface-change' to 'interface-changed'
- Add test for handle_sensor_value
- fix version test race condition
- Add tests for new inspecting_client connect-callback
  and factory functionality
Further addressing of PR43 comments
state-loop based InspectingClient

The inspecting client state is now managed by a single coroutine
running a 'state-machine' loop. Also yields on the user supplied
state-change callback, serialising the whole connect - inspect - sync
process.