Release 2.3.0 - Heater energy calculation

This release implements the following new features:

1. Add the device power and device energy into attributes - #25
2. Power of the heater should be accessible event if power management is not selected - #53 (thanks to @WarC0zes)

And also the following bugs:

1. Datetime in extra attributes are in UTC - #51
2. In certain case, temperature event are registred with an offset of one hour - #52 (thanks to @franckb)
3. Documentation is wrong - #55 (thanks to @impuR_Shozz)

No special things have to be done for installation.

Enjoy !