Add support for `proxy` and `without-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` option (along with a `without-ssl` flag for interacting with object stores without HTTPS).

Example usage:

```bash
$ ./gof3r put --proxy="http://localhost:8080" --without-ssl \
--path=/Users/hector/Downloads/opscode_ubuntu-14.04_chef-provisionerless.box \
--bucket=boxes --key=opscode_ubuntu-14.04_chef-provisionerless.box
2014/05/07 16:54:56 Duration: 22.434512988s

$ ./gof3r get --proxy="http://localhost:8080" --without-ssl \
--path=/Users/hector/Downloads/opscode_ubuntu-14.04_chef-provisionerless.box \
--bucket=boxes --key=opscode_ubuntu-14.04_chef-provisionerless.box
2014/05/07 16:59:18 Duration: 15.24604565s

$ md5 opscode_ubuntu-14.04_chef-provisionerless.box \
~/Downloads/opscode_ubuntu-14.04_chef-provisionerless.box
MD5 (opscode_ubuntu-14.04_chef-provisionerless.box) = 5d82709ae673c5a2f9500f9e68f2c4d6
MD5 (/Users/hector/Downloads/opscode_ubuntu-14.04_chef-provisionerless.box) = 5d82709ae673c5a2f9500f9e68f2c4d6
```

(This test was done against a local instance of Riak CS with the S3 API enabled.)