Merge pull request #77 from bens/master

MonadPlus instance for Proxy
Renamed `stdin` and `stdout` to `stdinLn` and `stdoutLn`
Fixed README.md to use `stdinLn` and `stdoutLn`
Added `print` to `Pipes.Prelude`
Polished tutorial
Added `readLn` to `Pipes.Prelude`