Change from ct/kwh to currency/kwh

[suaveolent]

This PR changes from the ct/kWh to €/£/mWh/kWh as preferred by Home Assistant (see #122).

[suaveolent]

@mampfes Did you have time to look at the PR? Do you need me to change anything?

[mampfes]

Thanks for your contribution. Sorry, I'm really busy at the moment. I didn't had time to review the PR completely, but I have some comments upfront (not sure if you want / can address them):

• I would call it price_per_kwh (instead of price_currency_per_xxx)
• The sources should return (only) price_per_kwh -> remove price_eur_per_mwh and price_ct_per_kwh
• As there are always mistakes between the price and the net-price sensor, I want to remove the price-sensor and keep the net price sensor (the one which includes surcharge and VAT), but rename it to price-sensor.
• It would be nice if the integration detects after an update that the old surcharge/VAT uses a different unit of measure and automatically convert it.

[suaveolent]

I will create a new PR with these changes.