Implement &| stdout and stderr piping

[hgoldstein]

Changes introduced by this pull request:

• &| is now part of the grammar as the stdout and stderr piping operator
• Redirecting stdout and stderr with &| is now supported on *nix and Redox

Drawbacks:

• The big crossplat module is a little messy. Perhaps moving all of the platform specific code to a separate file altogether would be better. It seems like Rust's pattern for this is something like:

#[cfg(windows), path = 'for_windows.rs']
mod some_functionality;

#[cfg(unix), path = 'for_unix.rs']
mod some_functionality;

...

Fixes: Closes #331

State: Ready to go, pending review.

[mmstick]

Thanks for the work! I'm about to land some big changes myself to the pipe logic -- process groups, signal masking, foreground management. Albeit, I've yet to figure out why my fg implementation isn't working. Perhaps has something to do with performing waitpid in a background thread, and then the foreground process needing to take control of and perform waitpid itself.

[mmstick]

And merged