Network API

[xoft]

The code can't be split without exposing cNetworkSingleton - most of the implementation relies on its internals. And I don't want to expose it, since it has libevent stuff in it, so we'd need to put libevent dependencies in the header.

I don't think the Network stuff is gonna grow much more. It's pretty much complete on the TCP side, and the UDP side can have a separate file of its own - after all, libevent doesn't have such a high level support for it anyway.