Fix origin IP address retrieval

This PR allows one to configure the name of the HTTP header in which the IP address from which a payment originated can be found.

The canonical default of `'REMOTE_ADDR'` is assumed in case there is no `ADYEN_IP_ADDRESS_HTTP_HEADER` setting specified in the Django project — which should be the case if any component of the HTTP stack transfers this information in another HTTP header (often, but not always `'HTTP_X_FORWARDED_FOR'`).
This docstring is cool, as you explained why you did that (as you said "we need"). Yet, it does not tell what the function actually did, like "return an IP address from header".

Also, maybe something more concise on the setting available, like "Header's key can be configured by the ADYEN_IP_ADDRESS_HTTP_HEADER setting, and `REMOTE_ADDR` is used as default.".

Don't get me wrong: I like your style and tone, but I like when I've just two lines to read to fully know what the function does, instead of a (still well written) full paragraph.