Introduce support for sockets in nonblocking mode

This is an initial draft implementation for #89 (closed).

There are currently 3 commits:

• The first introduces a raw module, with the RawConnection type, as small wrapper over a socket to handle buffering in non-blocking mode
• The second introduces the handshake module, containing two handcrafted state-machines for client-side & server-side handshakes
• The third commit switches out the internals of Connection to use RawConnection

Remaining to be done:

• Add more tests
• Bikeshed all the names
• Add an API for initializing connections in non-blocking mode