Add download.sh file

Relates to https://github.com/boot2docker/boot2docker-cli/issues/174.

This is my first draft to provide a way to install boot2docker via a script. As mentioned in the issue, this can be invoked like this:

```
bash -c "`curl -sL https://raw.githubusercontent.com/boo2docker/boo2docker-cli/master/download.sh`" && sudo mv boot2docker /usr/local/bin/boot2docker
```

Some caveats:
* I don't have a Windows system around, so not sure if it works on Windows. From what I understand, it should work under Cygwin.
* You might want to change `release.sh` so that it updates the version in the `download.sh` file.