Register the bzr repository type.

Hi Olivier, I got the following error while doing a `composer install`:

    Repository type is not registered: bzr

Here is my `composer.json` file:

```
{
	"name": "composer/test",
    "require": {
        "composer/test2": "@dev"
    },
    "repositories": [
        {
            "type": "bzr",
            "url":  "/home/jeremy/src/composer-test2"
        }
    ]
}
```

Indeed the repository type `bzr` isn't supported yet so here's my patch.

Cheers.