Use binary version of client

Instead of compiling. Far simpler. We'll soon have official
binaries, too:

https://github.com/dotcloud/docker/issues/1616
Show help when running without command
Use Vagrant image with Docker already installed

Removed upgrade system in preparation for automatic upgrades.
Exit immediately on errors
Add automatic upgrade system

If installed version is not the specified version, installs new
client and reprovisions VM.
Improve comments
Rewrite readme