Fix big messages, write failures, and delays

[andreivasiliu]

Fixed a few things:

• Messages larger than 256 bytes were being corrupted by the u8 message length
• Writes were being delayed because of blocking and incomplete writes (my client was lagging behind compared to the server)
• Reads were potentially discarding bytes (behavior on WouldBlock for read_exact is unspecified)
• Sprinkled some yield_now()'s in a few places to be nicer

Bumped version because send and send_bin's signatures became incompatible, although most people just need to add an .unwrap() on them.

Ideally this would use fully-blocking calls everywhere, although that for some reason results in massive delays on messages (I'm guessing it interferes with mpsc message passing or something? not sure yet). I might try to rewrite it to block, the code will be simpler.