Support finding links in S3 buckets

Hi all,

this patch permits the following use case

```
pip install --find-links=https://s3.amazonaws.com/my-bucket package
```

I implemented this by `get_page` discriminating against the domain name. It's just a rough patch, most likely incomplete but it works for me and I think some other users might find this useful too.

I couldn't run the test suite because already the tests for the develop branch were failing for me (something wrong with my setup I think)

Cheers