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.
Just place the file currencylayer.py in the /custom_components/sensor/ directory and activate using the following configuration
base: USD #optional
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