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.

This will also remove the implicit update() calls from device state changing methods such as set_brightness(). This allows executing multiple changes on a device without unnecessary I/O.

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

Closes #53, closes #61.