Refactored the Docker::Connection class.

This will allow for far more concise requests throughout the rest of the
gem.
Added a Docker::Util module.

This module is intended for all utilities that don't belong in any
specific class. As of right now, it only holds a `.parse_json` method.
Migrated Docker::Model and Docker::Container over to using the new
Connection class.
Updated Docker::Image to use the new Connection class.
Migrated the Docker module to use the new Connection; misc cane fixes.
Version bump 1.1.0
Added `Docker.validate_version!`.

This method raises an error if Docker is not the right version.
Merge pull request #13 from swipely/more-centralized-requests

More centralized requests