Implement basic socket layer netcode

[hnisula]

• [x] Implement address object (IPv4).
• [x] Implement Socket class with minimal functionality (creation, binding, non-blocking config, send/receive, IPv4).
• [x] Come up with (at least a beginning of) a protocol for this.
• [x] Handle sending of packets that need fragmentation (due to packet size limits on the internet).
• [x] Handle receiving fragmented packets.
• [x] Create simple test program for manually sending packets through a socket.
• [x] Create test program for sending large packets that need to be fragmented.

Closes #44.

[hnisula]

Using some kind of checksum/CRC in the protocol header seems a bit redundant as the future encryption would validate the same thing (and more) anyway. At least skip it for now.