Authentication via API key

[sockenklaus]

Offer some kind of authentication:

1. On server side you have to write a key into the config.yml or generate it via ingame command.
2. On client side you have to transmit a matching key as a url parameter to authenticate yourself.

Nice to have:

• Use different keys for different usecases so key (a) can only request users.xml, key (b) is only allowed to request userstats.xml and key (c) can access all xml files for example.
• If authentication is required for a xml file and a client tries to access it without a (valid?) key, the admin receives an ingame message and has the possibility to automatically generate a key for the requested file and hand it over to the requesting client.