Support interactive stdin/stdout streams

[dylanmckay]

This adds support for streaming stdin, stderr, and stdout independently
to a running container.

The underlying API is futures-based, meaning the code is implemented
asynchronously. A synchronous API is also exposed, which is implemented
by simply waiting on the asynchronous API futures.

This also modifies the existing Tty logic so that the storage type of
the data is a Vec rather than a String. This is also how the Rust
standard library persists data from the standard streams. In my
particular application, I'm using stdin/stdout as the communication
method between a container a host application. In it, a byte-based protocol is
used.

Streaming works by performing a TCP upgrade; upgrading a higher-level
HTTP connection to a lower-level TCP byte stream upon agreement with the
server. Docker will automatically upgrade HTTP container log requests to
TCP byte streams of a custom std{in,out,err} multiplexing protocol if
the client requests it with the 'Connection: Upgrade' header.

How did you verify your change:

I've got a project that implements a nontrivial byte-based protocol to send command/reply frames from the host app to the container over stdout/stdin. I can successfully parse, send, and process frames in both directions asynchronously with this patch.

What (if anything) would need to be called out in the CHANGELOG for the next release:

• The Container::logs function now returns a tty::Chunk type containing raw bytes, rather than a TtyLine object containing a UTF-8 string.
• The standard streams of container logs can now be independently managed and communicated with

[abusch]

It might be better to return a failed Future here, instead of panicking?

[dylanmckay]

Good idea

[dylanmckay]

Fixed

[dylanmckay]

I've also added Chunk::as_string, Chunk::as_string_lossy accessors, and updated the examples to use them.

Tests pass on my machine, I can compile all examples, and all tests pass.

One of the existing PRs to fix the existing rustfmt error in shiplift/master need to be merged before CI will pass for this change. I've included it in one of my smaller PRs, #134, but it looks like #137 also does it.

[softprops]

apologize for the slow feedback cycle. I've been on vacation. there are some changes I think that need to happen in my travis build. I think pr check errors are unrelated to your changes