Enable test-integration-cli for Windows platform

This is more steps toward functional `test-integration-cli` for Windows client-side. How to run under Cygwin:

```
export CYGWIN=winsymlinks:lnk                          # Use simple .lnk for symlinks
export DOCKER_CLIENTONLY=1
export DOCKER_TEST_HOST=tcp://1.2.3.4:5678
project/make.sh binary test-integration-cli
```

Known issues:
- [Many tests fail](https://gist.github.com/icecrime/c82ba2ffe7193275ff02), most likely a majority because of the test implementation itself
- Coverage is not measured: I'm guessing this is another cygpath issues leading to coverfiles not being produced
- This doesn't work with [git-provided `bash.exe`](http://stackoverflow.com/questions/3983484/does-the-current-version-of-msysgit-support-process-substitution): perhaps a bash expert can bring his expertise here

I believe this is worth merging as it provides a reasonable base for people to start fixing individual failing tests.

-----

Update: how to set up a test environment on Windows:

1. You'll need `bash.exe`, and unfortunately the one provided with git installer [won't work](http://stackoverflow.com/questions/3983484/does-the-current-version-of-msysgit-support-process-substitution). I chose to use [Cygwin](https://www.cygwin.com/) out of habit, but there's probably other (and perhaps better) choices out there.
2. Because Windows doesn't support Docker daemon yet, you'll have to run a daemon somewhere to connect to. Remember to launch with the daemon with the appropriate `-H` directive allowing your Windows box to access it remotely.
3. Launch Cygwin (the shell defaults to bash), `cd` to Docker source directory, and run:
  * `export DOCKER_CLIENTONLY=1`: indicates to the build scripts that you're running on a "client-only" platform
  * `export DOCKER_TEST_HOST=tcp://<docker_host>:<docker_port>`: gives the build scripts the address of the remote Docker host
  * `project/make.sh binary test-integration-cli`

Troubleshooting: the `binary` target should produce both a `docker-$VERSION.exe` binary inside `./bundles/$VERSION/binary/` and a `docker.exe.lnk` next to it. I encountered issues where the docker command was not recognized even though the link was in my path: this was due to Cygwin producing [other types of links](https://cygwin.com/cygwin-ug-net/using.html#pathnames-symlinks), and was fixed using `export CYGWIN=winsymlinks:lnk`.
Just wanted to bust some stones, especially @jfrazelle's.