Add support for `proxy_uri` and `use_ssl`

Most private object storage solutions support direct and proxy configurations. For clients interacting with object stores in a direct configuration, overriding `endpoint` works. In order to support object stores in a proxy configuration, this commit adds a `proxy_uri` option (along with a `use_ssl` flag for object stores without HTTPS enabled).

Example usage:

```bash
$ deb-s3 upload --access-key-id "Y6W_BQNKFAS9XMN43-GJ" --secret-access-key "S8PQ6lSQ8q6GIauyQ3g2yozJRST3RpagymBkVg==" \
    --proxy-uri "http://localhost:8080" --use-ssl=false --bucket joker riak-ee_1.4.8-1_amd64.deb
>> Retrieving existing manifests
>> Examining package file riak-ee_1.4.8-1_amd64.deb
>> Uploading packages and new manifests to S3
   -- Transferring pool/r/ri/riak-ee_1.4.8-1_amd64.deb
   -- Transferring dists/stable/main/binary-amd64/Packages
   -- Transferring dists/stable/main/binary-amd64/Packages.gz
   -- Transferring dists/stable/Release
>> Update complete.
$ deb-s3 verify --access-key-id "Y6W_BQNKFAS9XMN43-GJ" --secret-access-key "S8PQ6lSQ8q6GIauyQ3g2yozJRST3RpagymBkVg==" \
    --proxy-uri "http://localhost:8080" --use-ssl=false --bucket joker
>> Retrieving existing manifests
>> Checking for missing packages in: stable/main amd64
>> Checking for missing packages in: stable/main armel
>> Checking for missing packages in: stable/main i386
>> Checking for missing packages in: stable/main all
```

(This test done against a local instance of Riak CS with the S3 API enabled.)