Add heater

[JELoohuis]

• Added support for the heater category of devices.
  Since my test device had limited functionality the following capabilities were implemented and tested:

  • onoff (switch)
  • target_temperature (temp_set)
  • measure_temperature (temp_current)
  • measure_power (work_power)

  The eco-mode (mode_eco) and child_lock (lock) capability were added untested,
  so it would be nice to get feedback on them from users with devices that support them.

  The level instruction could not be tested and was not added,
  as it is unclear whether all devices that support it use all three levels,
  which would dictate the type of Homey capability used.

May resolve:

• Request Device: Heater #10
• Request Device: Action Infrared panel heater #16

[MerelHudepohl]

@JELoohuis i'm gonna put this in test so we might get some feedback. Once the icons are available please add them.