Remote repositories

Hello everyone!

I think it would be great to can build remote repositories. For example, storing files in AWS, GCloud, ssh,... This would provide easy storage, without needing configurate custom services to serve distribution packages. Actually, if you want to do this, you have to store a full repository and sync against your target storage service.

It could be like:

```
# satis-config.json
{
"name": "My composer repository at AWS",
"homepage": "https://my-satis-test-repository.s3-eu-west-1.amazonaws.com",
...
"archive": {
"directory": "dist",
"prefix-url": "https://my-satis-test-repository.s3-eu-west-1.amazonaws.com"
}
}
```

```
$ satis build satis-config.json s3://my-satis-test-repository`
Scanning packages
Creating remote downloads in 's3://my-satis-test-repository/dist'
...
Wrote packages json s3://my-satis-test-repository/include/all$11a4509fabb4253e23ad6d3b04f27182fc251f40.json
Writing s3://my-satis-test-repository/packages.json
Writing web view
```

This would require configure some authentication methods that could be located in $HOME/.composer/auth.json

What do you think?