Add Search image capabilities

Hello guys,
As for #17 implementation of SearchImages.
Does the Docker API return "is_official" or "isofficial"? If you add a new test case, where is_official is true, this will break. Also, "is_trusted" should be "is_automated", right?

You'll need to change the json and yaml structs to use "is_offical" and "is_automated". Thank you again!