Just wanted to share another exchange rate sensor based on http://currencylayer.com. This one also offers 1000 free requests per month, which should be enough for casual use.
https://github.com/arsaboo/HASS/tree/master/custom-components/currencylayer
Just place the file currencylayer.py in the /custom_components/sensor/ directory and activate using the following configuration
sensor:
platform: currencylayer
api_key: your_api_key
base: USD #optional
quote: EUR
name: USDEUR #optional
Unfortunately, I have hit a wall with committing these sensors to the official repository. So, here they are for the time being 