Optimize I/O access (#59)

* Optimize I/O access

A single update() will now fetch information from all interesting modules,
including the current device state and the emeter information.

In practice, this will allow dropping the number of I/O reqs per homeassistant update cycle to 1,
which is paramount at least for bulbs which are very picky about sequential accesses.
This can be further extend to other modules/methods, if needed.

Currently fetched data:
* sysinfo
* realtime, today's and this months emeter stats

New properties:
* emeter_realtime: return the most recent emeter information, update()-version of get_emeter_realtime()
* emeter_today: returning today's energy consumption
* emeter_this_month: same for this month

Other changes:
* Accessing @requires_update properties will cause SmartDeviceException if the device has not ever been update()d
* Fix __repr__ for devices that haven't been updated
* Smartbulb uses now the state data from get_sysinfo instead of separately querying the bulb service
* SmartStrip's state_information no longer lists onsince for separate plugs
* The above mentioned properties are now printed out by cli
* Simplify is_on handling for bulbs

* remove implicit updates, return device responses for actions, update README.md instructions. fixes #61

kasa/tests/test_cli.py

@@ -66,7 +66,7 @@ async def test_emeter(dev: SmartDevice, mocker):
         assert "Device has no emeter" in res.output
         return
 
-    assert "Current State" in res.output
+    assert "== Emeter ==" in res.output
 
     monthly = mocker.patch.object(dev, "get_emeter_monthly")
     res = await runner.invoke(emeter, ["--year", "1900"], obj=dev)