Update README according to current implementation

Updated the README to reflect that we now are working against 1.0.* of the docker project.

With the latest release, `Docker::Image#insert` was removed from the API altogether, and we added support for the pause and unpause feature.

We're missing the ability to load and unload the images in their docker format as a tarball which was released in v1.10 or v1.11.

The only other thing I could see missing was supplying a signal to the kill endpoint, which is currently open in #136.

@bfulton @nahiluhmot