Load / Save Images

Hi,

I was updating/porting doguestry (http://github.com/ingenieux/dogestry), which is a kludge to avoid dealing with private internal docker repositories [more details here](http://michaelneale.blogspot.com.br/2014/03/go-and-alternative-docker-registry.html), and I missed the commands for dealing with raw image import/export (via tarfiles), so I added it back.

Let me know if its okay for you guys. go-dockerclient has been amazing to me so far, and I'd like to thank you for this!

(I even managed to use it directly from windows into boot2docker for local docker build! \o/)
I'm wondering whether having an ExportImageOptions struct would be better. Just for making it easier to add new parameters in the future, in case Docker expands the API. I believe the applies to LoadImage.