Add support for HTTP_PROXY and without-ssl flag

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 support for the `HTTP_PROXY` environmental variable (along with a `without-ssl` flag for interacting with object stores without HTTPS).

Example usage:

```bash
$ HTTP_PROXY="http://localhost:8080" ./gof3r put --without-ssl \
--path=/Users/hector/Downloads/ubuntu-14.04-server-amd64.iso \
--bucket=isos --key=ubuntu-14.04-server-amd64.iso
2014/05/11 22:47:09 Duration: 28.509582349s

$ HTTP_PROXY="http://localhost:8080" ./gof3r get --without-ssl \
--path=ubuntu-14.04-server-amd64.iso \
--bucket=isos --key=ubuntu-14.04-server-amd64.iso
2014/05/11 22:48:04 Duration: 16.278402355s

$ md5 ubuntu-14.04-server-amd64.iso ~/Downloads/ubuntu-14.04-server-amd64.iso
MD5 (ubuntu-14.04-server-amd64.iso) = 01545fa976c8367b4f0d59169ac4866c
MD5 (/Users/hector/Downloads/ubuntu-14.04-server-amd64.iso) = 01545fa976c8367b4f0d59169ac4866c
```

(This test was done against a local instance of Riak CS with the S3 API enabled.)