Full Magento base install via composer

Hi,

By adding the `magento-base` package type, you could do something as follows in a Magento base install package:

```json
{
    "name":"vendor/magento-base-install",
    "description":"Magento CE Edition",
    "type":"magento-base",
    "require": {
         "composer/installers": "~1.0"
    },
    "extra": {
        "installer-name": "htdocs"
    }
}
```
In the consumer (root) package, you could basically start a whole Magento install, as follows:

```json
{
    "minimum-stability":"dev",
    "require": {
        "vendor/magento-base-install":"1.8.1.0",
    },
    "repositories": [
        {
            "type":"vcs",
            "url":"git@github.com:sdesmet/magento-base-install.git"
        }
    ]
}
```

And just run `composer create-project`.

This approach would also work fine with other initiatives in the Magento community, e.g. https://github.com/magento-hackathon/magento-composer-installer.