This change adds OpenSSL as an explicit dependency to allow creation,
verification, and management of private and public keys for player
account verification. The same keys will be used to sign and verify
client/server connection messages.

The crypto part of client/server communication and player account
handling will require extensive use of crypto functions, which should be
implemented in a central place.

The SOCI database backend as well as the general database interface got
functions for storing and managing data. This includes creation of
arbitrary entities, adding properties to them, and deleting either of
them again. The only thing missing in this one is the retrieval part,
which will be done next.