Merge Amazon S3 driver

As stated in #46 , the S3 driver now passes all the tests (at least on my computer).

It should be ready for merge.
I'm not a big fan of `\` in strings, I prefer using several strings (Python concatenate them).  Plus in Onitu, we use `"` when it's a sentence (and `'` for dict keys for instance).

I would write it like that : 
```python
raise DriverError(
    "The given root '{}' isn't a directory on the '{}' bucket. It "
    "has to be an empty file.".format(
        plug.options['root'], plug.options['bucket'])
    )
)
```