Progress from calls such as PullImage

[ibuildthecloud]

I'd like to print progress of the PullImage. I'm willing to do the PR but would like some guidance on the approach. Do we break the current API, new API? Does the API accept a chan and blocks, should it return immediatly and not block..... Just give me a method signature and I'll implement it :)

[ptheunis]

Would love this feature aswell, also in that api can we return the ID of the newly pulled image?

[odewahn]

Same here -- I'm working on a GUI tool that lets you pull images and I'd love to be able to show a percent complete / progress bar.