Add global command to be able to require/install/update global packages

Replaces #1813 and fixes #55

e.g. this installs php-cs-fixer globally (you just have to add the `~/.composer/vendor/bin` dir to your PATH:

```
$ composer global require fabpot/php-cs-fixer:dev-master
```

It is really just a way to make something that was already possible a tad easier and more "standard".

Feedback welcome.